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1.2.08

Microbiology Questions asked in saudi madical council exame

Micro
1.
The latest and most effective therapy for AIDS patients includes azidothymidine (AZT), dideoxyinosine (DDI), and saquinavir or similar agents. Use of these three drugs would inhibit which of the following viral processes?
A. RNase, DNase
B. gp120 formation
C. p24 antibody expression
D. All membrane synthesis
E. Reverse transcriptase, protease

The answer is: E

The advent of triple therapy or a therapeutic "cocktail" has had a marked effect on AIDS patients. The combination of drugs work together as reverse transcriptive inhibitors and a protease inhibitor. Patients improve rapidly, their CD4 lymphocyte counts increase, and their HIV viral load is drastically reduced, often to <50>


2.
A tube of monkey kidney cells is inoculated with nasopharyngeal secretions. During the next 7 days, no cytopathic effects (CPEs) are observed. On the eighth day, the tissue culture is infected accidentally with a picornavirus; nevertheless, the culture does not develop CPEs. Which of the following is the most likely explanation of this phenomenon?

A. The nasopharyngeal secretions contained hemagglutinins
B. The nasopharyngeal secretions contained rubella virus
C. Picornavirus does not produce CPEs
D. Picornavirus does not replicate in monkey kidney cells
E. Monkey kidney cells are resistant to CPEs

The answer is: B

Rubella virus does not produce cytopathic effects (CPEs) in tissue-culture cells. Moreover, rubella-infected cells challenged with a picornavirus are resistant to subsequent infection and thus would not exhibit CPEs. Monkey kidney cells infected only with picornavirus would show CPEs.

3.
Which of the following is the most sensitive test for the diagnosis of herpes simplex (HSV) meningitis in a newborn infant?
A. HSV IgG antibody
B. HSV polymerase chain reaction (PCR)
C. HSV culture
D. Tzanck smear
E. Cerebrospinal fluid (CSF) protein analysis

The answer is: B

HSV meningitis or encephalitis is difficult to diagnose by laboratory tests as there is a low titer of virus present in the CSF. Neonatal HSV infects the child during the birth process. While culture, Tzanck smear, and even antibody tests may be useful in adults, particularly those with HSV-rich lesions, they are not useful for CSF testing. Only PCR is sensitive enough to detect HSV DNA in the CSF. Once diagnosed rapidly, HSV encephalitis or meningitis can be treated with acyclovir.

4.
One of the most common sexually transmitted diseases that may lead to cervical carcinoma is caused by which of the following viruses?
A. Cytomegalovirus
B. Papillomavirus
C. Epstein-Barr virus
D. Herpes simplex virus
E. Adenovirus

The answer is: B

Human papillomavirus (HPV) is the cause of genital warts. It is one of the most pervasive of all the sexually transmitted diseases. There is no specific cure or vaccine. There are multiple serotypes of papillomavirus and some serotypes are linked to cervical cancer. New techniques for molecular diagnosis of HPV show promise for rapid and sensitive detection and perhaps more aggressive treatment.

5.
Meningitis is characterized by the acute onset of fever and stiff neck. Aseptic meningitis may be caused by a variety of microbial agents. During the initial 24 h of the course of aseptic meningitis, an affected person’s cerebrospinal fluid is characterized by which of the following?
A. Decreased protein content
B. Elevated glucose concentration
C. Lymphocytosis
D. Polymorphonuclear leukocytosis
E. Eosinophilia

The answer is: D

Aseptic meningitis is characterized by a pleocytosis of mononuclear cells in the cerebrospinal fluid; polymorphonuclear cells predominate during the first 24 h, but a shift to lymphocytes occurs thereafter. The cerebrospinal fluid of affected persons is free of culturable bacteria and contains normal glucose and slightly elevated protein levels. Peripheral white blood cell counts usually are normal. Although viruses are the most common cause of aseptic meningitis, spirochetes, chlamydiae, and other microorganisms also can produce the disease.

6.
Kuru is a fatal disease of certain New Guinea natives and is characterized by tremors and ataxia; Creutzfeldt-Jakob disease (CJD) is characterized by both ataxia and dementia. These diseases are thought to be caused by which of the following?
A. Slow viruses
B. Cell wall–deficient bacteria
C. Environmental toxins
D. Prions
E. Flagellates

The answer is: D

Kuru and Creutzfeldt-Jakob disease (CJD) are similar but not identical diseases with very different epidemiology. Kuru is prevalent among certain tribes in New Guinea who practiced ritual cannibalism by eating the brains of the departed. CJD is found worldwide and has been transmitted by corneal transplants and in pituitary hormone preparations. There is some association between CJD and Mad Cow Disease in England. Prions are unconventional selfreplicating proteins, sometimes called amyloid. It is now thought that CJD, Kuru, and animal diseases such as scrapie, visna, and bovine spongiform encephalopathy (Mad Cow Disease) are caused by prions.

7.
An infant, seen in the ER, presents with a fever and persistent cough. Physical examination and a chest x-ray suggest pneumonia. Which of the following is most likely the cause of this infection?
A. Rotavirus
B. Adenovirus
C. Coxsackievirus
D. Respiratory syncytial virus
E. Rhinovirus

The answer is: D

Respiratory syncytial virus (RSV) is the most important cause of pneumonia and bronchiolitis in infants. The infection is localized to the respiratory tract. The virus can be detected rapidly by immunofluorescence on smears of respiratory epithelium. In older children, the infection resembles the common cold. Aerosolized ribavirin is recommended for severely ill hospitalized infants.

8.
Which one of the following viruses may be human tumor virus?
A. Epstein-Barr virus (EBV)
B. HIV
C. Papillomavirus
D. Varicella-zoster virus (VZV)
E. Herpes simplex virus, type 2 (HSV)

The answer is: C

Only two human viruses have been confirmed as human tumor viruses. They include human T-cell lymphoma/leukemia virus (HTLV) and papillomavirus. Others, such as EBV, HSV, and hepatitis B and C, have been implicated as tumor viruses. The virus that causes chicken pox (VZV) is not know to be oncogenic.

9.
There is considerable overlap of signs and symptoms seen in congenital and perinatal infections. In a neonate with "classic" symptoms of congenital cytomegalovirus (CMV) infection, which one of the following tests would be most useful in establishing a diagnosis?
A. CMV IgG titer on neonate’s serum at birth
B. CMV IgG titer on mother’s serum at birth of infant
C. CMV IgM titer on neonate’s serum at birth and at 1 month of age
D. Total IgM on neonate’s serum at birth
E. Culture of mother’s urine

The answer is: C

(See figure below.) Presently, cytomegalovirus (CMV) is the most common cause of congenital and perinatal viral infections. Culture of the virus is a sensitive diagnostic technique; in the case of a neonate with classic symptoms, serum samples from the mother and neonate are obtained at birth. The IgM antibody titer in the infant’s serum should be higher than the mother’s titer, but they may be similar. For this reason, another sample from the infant at 1 month of age is tested simultaneously with the initial sample. The results should indicate a rise in IgM titer. Measurement of total IgM in the infant’s sera at birth is nonspecific and may show false-negative and false-positive reactions.

Cytomegalovirus-infected human embryonic fibroblasts stained with fluorescein-labeled monoclonal antibody to early nuclear antigen (×1000).

10.
Interferon, a protein that inhibits viral replication, is produced by cells in tissue culture when the cells are stimulated with which of the following?
A. Botulinum toxin
B. Synthetic polypeptides
C. Viruses
D. Chlamydiae
E. Gram-positive bacteria

The answer is: C

Interferon is a protein that alters cell metabolism to inhibit viral replication. It induces the formation of a second protein that interferes with the translation of viral messenger RNA. Production of interferon has been demonstrated when cells in tissue culture are challenged with viruses, rickettsiae, endotoxin, or synthetic double-stranded polynucleotides. Interferon confers species-specific, not virus-specific, protection for cells.

11.
Which one of the following viruses would be most likely to establish a latent infection?
A. Adenovirus
B. Measles virus
C. Influenza virus
D. Parvovirus
E. Coxsackievirus group B

The answer is: A

While the herpesviruses (HSV, CMV, VZV) are all well known for latency, adenovirus can also form a latent infection in the lymphoid tissue. In 50 to 80% of surgically removed tonsils or adenoids, adenovirus can be cultured. The virus has also been cultured from mesenteric lymph nodes, and, in rare cases, viral DNA has been detected in peripheral lymphocytes. Recurrent illness usually does not arise from these latent infections; however, activation can occur in the immunosuppressed.

12.
Chicken pox is a common disease of childhood. It is caused by which of the following viruses?

A. Cytomegalovirus
B. Rotavirus
C. Varicella-zoster virus
D. Adenovirus
E. Papillomavirus

The answer is: C

Varicella-zoster virus is a herpesvirus. Chicken pox is a highly contagious disease of childhood that occurs in the late winter and early spring. It is characterized by a generalized vesicular eruption with relatively insignificant systemic manifestations.

Adenovirus has been associated with adult respiratory disease among newly enlisted military troops. Crowded conditions and strenuous exercise may account for the severe infections seen in this otherwise healthy group.

Papillomavirus is one of two members of the family Papovaviridae, which includes viruses that produce human warts. These viruses are host-specific and produce benign epithelial tumors that vary in location and clinical appearance. The warts usually occur in children and young adults and are limited to the skin and mucous membranes.


Rotavirus is worldwide in distribution and has been implicated as the major etiologic agent of infantile gastroenteritis. Infection with this virus varies in its clinical presentation from asymptomatic infection to a relatively mild diarrhea to a severe and sometimes fatal dehydration. The exact mode of transmission of this infectious agent is not known. Because of severe side effects, the rotavirus vaccine has been recalled and is temporarily unavailable.


Infectious mononucleosis caused by cytomegalovirus (CMV) is clinically difficult to distinguish from that caused by Epstein-Barr virus. Lymphocytosis is usually present with an abundance of atypical lymphocytes. CMV-induced mononucleosis should be considered in any case of mononucleosis that is heterophil-negative and in patients with fever of unknown origin.

13.
Human warts are not only cosmetically unsightly but may lead to cancer of the cervix. They are caused by which one of the following viruses?
A. Cytomegalovirus
B. Rotavirus
C. Varicella-zoster virus
D. Adenovirus
E. Papillomavirus

The answer is: E

14.
A child has mononucleosis-like symptoms yet the test for mononucleosis and the EBV titers are negative. Which of the following is one cause of heterophile-negative mononucleosis?
A. Cytomegalovirus
B. Herpes simplex virus
C. Varicella-zoster virus
D. Adenovirus
E. Coxsackievirus

The answer is: A

Varicella-zoster virus is a herpesvirus. Chicken pox is a highly contagious disease of childhood that occurs in the late winter and early spring. It is characterized by a generalized vesicular eruption with relatively insignificant systemic manifestations.


Adenovirus has been associated with adult respiratory disease among newly enlisted military troops. Crowded conditions and strenuous exercise may account for the severe infections seen in this otherwise healthy group.


Papillomavirus is one of two members of the family Papovaviridae, which includes viruses that produce human warts. These viruses are host-specific and produce benign epithelial tumors that vary in location and clinical appearance. The warts usually occur in children and young adults and are limited to the skin and mucous membranes.


Rotavirus is worldwide in distribution and has been implicated as the major etiologic agent of infantile gastroenteritis. Infection with this virus varies in its clinical presentation from asymptomatic infection to a relatively mild diarrhea to a severe and sometimes fatal dehydration. The exact mode of transmission of this infectious agent is not known. Because of severe side effects, the rotavirus vaccine has been recalled and is temporarily unavailable.


Infectious mononucleosis caused by cytomegalovirus (CMV) is clinically difficult to distinguish from that caused by Epstein-Barr virus. Lymphocytosis is usually present with an abundance of atypical lymphocytes. CMV-induced mononucleosis should be considered in any case of mononucleosis that is heterophil-negative and in patients with fever of unknown origin.

15.
Malaise and fatigue with increased "atypical" lymphocytes and a reactive heterophil antibody test is most commonly caused by which of the following?

A. Toxoplasma
B. Borrelia burgdorferi
C. Epstein-Barr virus
D. Parvovirus
E. Rubella virus

The answer is: C

Epstein-Barr virus (EBV) is a herpesvirus that causes a number of syndromes; the most common is infectious mononucleosis. It is a ubiquitous enveloped DNA virus. Only one serotype of EBV has been recognized, although molecular methods have reorganized a number of genotypes of EBV.

Infectious mononucleosis is an acute disease most commonly seen in younger people. It is characterized by a proliferation of lymphocytes, lymph node enlargement, pharyngitis, fatigue, and fever. Infection in young children is usually either asymptomatic or characteristic of an acute upper respiratory infection. Diagnosis is usually made by a positive heterophil test. Heterophil antibodies are those that occur in one species (human) and react with antigens of a different species. The heterophil test may be insensitive (30 to 60%) in children. Definitive diagnosis is made by detection of antibodies to EBV components.


EBV causes a variety of other syndromes including Burkitt’s lymphoma, the most common childhood cancer in Africa, and nasopharyngeal carcinoma, commonly seen in China.

Similar mononucleosis-like diseases are caused by cytomegalovirus (CMV) and Toxoplasma gondii, a parasite. CMV causes fewer than 10% of infectious mononucleosis-like diseases. CMV "mono" is primarily characterized by fatigue. Congenital infection with CMV almost always causes serious sequelae, such as retardation and hearing loss. T. gondii also causes a variety of clinical problems, among them encephalitis in AIDS patients and food poisoning from the ingestion of raw meat. Although CMV and T. gondii are relatively rare causes of infectious mononucleosis, they must be ruled out, particularly when EBV tests are nonreactive.

16.
Which of the following viruses causes progressive multifocal leukoencephalopathy (PML), a disease causing demyelination in the central nervous system?
A. Human papillomavirus
B. West Nile virus
C. Tick-borne encephalitis virus
D. Polyomavirus
E. SSPE

The answer is: D

Human papillomaviruses (HPV) are the causative agents of cutaneous warts as well as proliferative squamous lesions of mucosal surfaces. Although most infections by human papillomavirus are benign, some undergo malignant transformation into in situ and invasive squamous cell carcinoma. Both HPV and polyomavirus have icosahedral capsids and DNA genomes. JC virus, a polyomavirus, was first isolated from the diseased brain of a patient with Hodgkin’s lymphoma who was dying of progressive multifocal leukoencephalopathy (PML). This demyelinating disease occurs usually in immunosuppressed persons and is the result of oligodendrocyte infection by JC virus. JC virus has also been isolated from the urine of patients suffering from demyelinating disease. Cryotherapy and laser treatment are the most popular therapies for warts, although surgery may be indicated in some cases. At the present time, there is no effective antiviral therapy for treatment of infection with polyomavirus or HPV. West Nile virus is an arbovirus. While prevalent in Europe, Africa, and the Middle East, it was not seen in the United States until the summer of 1999. It is transmitted by mosqitoes and birds, especially crows; these animals are a reservoir. WNV causes a rather mild encephalitis in humans, the exception being older patients or those who may be immunocompromised.

17.
Which of the following is the leading cause of bronchiolitis and community-acquired pneumonia in infants?
A. Measles virus
B. Influenza virus
C. Respiratory syncytial virus
D. Parainfluenza virus
E. Adenovirus

The answer is: C

Orthomyxoviruses and paramyxoviruses are RNA viruses that contain a single-stranded RNA genome. The influenza viruses belong to the orthomyxoviruses. They cause acute respiratory tract infections that usually occur in epidemics. Isolated strains of influenza virus are named after the virus type (influenza A, B, or C) as well as the host and location of initial isolation, the year of isolation, and the antigenic designation of the hemagglutinin and neuraminidase. Both the hemagglutinin and neuraminidase are glycoproteins under separate genetic control, and because of this they can and do vary independently. The changes in these antigens are responsible for the antigenic drift characteristic of these viruses. The paramyxoviruses include several important human pathogens (mumps virus, measles virus, respiratory syncytial virus, and parainfluenza virus). Both paramyxoviruses and orthomyxoviruses possess an RNA-dependent RNA polymerase that is a structural component of the virion and produces the initial RNA. Respiratory syncytial viruses (RSV) are not related to the paramyxoviruses. They are 150-nm single-stranded RNA viruses. There are 2 antigen groups, A and B, which play no role in diagnosis and treatment. While the overall mortality is 0.5%, at-risk groups may be 25 to 35% mortality if untreated. Some parainfluenza virus infections (type 3) may be indistinguishable from RSV, but most parainfluenza infections produce a laryngotracheobronchitis known as croup.

18.
A patient with a peptic ulcer was admitted to the hospital and a gastric biopsy was performed. The tissue was cultured on chocolate agar incubated in a microaerophilic environment at 37°C for 5 to 7 days. At 5 days of incubation, colonies appeared on the plate and were curved, Gram-negative rods, oxidase-positive. Which of the following is the most likely identity of this organism?
A. Campylobacter jejuni
B. Vibrio parahaemolyticus
C. Haemophilus influenzae
D. Helicobacter pylori
E. Campylobacter fetus

The answer is: D

Helicobacter pylori was first recognized as a possible cause of gastritis and peptic ulcer by Marshall and Warren in 1984. This organism is readily isolated from gastric biopsies but not from stomach contents. It is similar to Campylobacter species and grows on chocolate agar at 37°C in the same microaerophilic environment suitable for C. jejuni (Campy-Pak or anaerobic jar [Gas Pak] without the catalyst). H. pylori, however, grows more slowly than C. jejuni, requiring 5 to 7 days incubation. C. jejuni grows optimally at 42°C, not 37°C, as does H. pylori.

19.
At a church supper in Nova Scotia, the following meal was served: baked beans, ham, coleslaw, eclairs, and coffee. Of the 30 people who attended, 4 senior citizens became ill in 3 days; 1 eventually died. Two weeks after attending the church supper, a 19-year-old girl gave birth to a baby who rapidly became ill with meningitis and died in 5 days. Epidemiologic investigation revealed the following percentages of people who consumed the various food items: baked beans, 30%; ham, 80%; coleslaw, 60%; eclairs, 100%; and coffee, 90%. Microbiologic analysis revealed no growth in the baked beans, ham, or coffee; many Gram-positive beta-hemolytic, short, rod-shaped bacteria in the coleslaw; and rare Gram-positive cocci in the eclairs. Which of the following is the most likely cause of this outbreak?
A. Staphylococcus aureus
B. Listeria
C. Clostridium perfringens
D. Clostridium botulinum
E. Nonmicrobiologic

The answer is: E

There have been a number of outbreaks of food poisoning caused by Listeria monocytogenes. Listeria is a common inhabitant of farm animals and can be readily isolated from silage, hay, and barnyard soil. Humans at the extremes of age are most susceptible to Listeria infection but only recently has food been implicated as a vehicle. In the outbreak in Nova Scotia, it is likely that the cabbage used for the coleslaw was fertilized with animal droppings and not properly washed prior to consumption. Major Listeria outbreaks associated with cheese have been seen in the United States and most likely have originated from contaminated milk. Epidemiologic investigation often will provide data on attack rates in such outbreaks. The eventual solution of the problem always lies in a combination of epidemiologic, microbiologic, and clinical information. For example, in the Nova Scotia case, it should not be assumed that the eclairs were the culprit based on the fact that everyone ate them.

20.
Group B streptococcus sepsis in an infant is preventable. Which one of the following procedures is most likely to reduce the incidence of group B streptococcal disease?
A. Intrapartum antibiotic treatment
B. Use of a polysaccharide vaccine
C. Screening of pregnant females in the last trimester
D. Identification of possible high-risk births
E. Screening of pregnant females at the first office visit, usually during the first trimester

The answer is: A

The incidence of group B streptococcal disease (GBS) is 1–3 cases per 1000 births. Neonates acquire the disease during birth from mothers who harbor the organism. Risk factors include prematurity, premature rupture of membranes, and group B streptococcal carriage. The Gram's stain of cerebrospinal fluid is a rapid test for GBS disease. Although sensitive, the Gram's stain requires experience to differentiate these streptococci from other Gram-positive cocci. Latex tests for GBS antigen are also available, but sensitivity in CSF is not significantly higher than the Gram's stain. GBS can be reduced by intrapartum administration of penicillin. Experimentally, GBS polysaccharide vaccines have also been used. Screening pregnant females early in pregnancy probably offers little advantage because of the possible acquisition of GBS late in the pregnancy. There has been speculation concerning the pathogenesis of GBS. These include failure to activate complement pathways and immobilization of polymorphonuclear leukocytes (PMNs) due to the inactivation of complement C5A, a potent chemoattractant. While GBS is relatively more resistant to penicillin than group A streptococci, the great majority of GBS isolates are still penicillin-susceptible. An aminoglycoside such as gentamicin may be added to GBS treatment regimens due to the relative reduced susceptibility of some strains.

21.
There has been much speculation on the pathogenesis of group B streptococcal disease in the neonate. Which of the following is the most likely pathogenic mechanisms?
A. Complement C5a, a potent chemoattractant, activates PMNs
B. The streptococci are resistant to penicillin
C. The alternative complement pathway is activated
D. In the absence of specific antibody, opsonization, phagocyte recognition, and killing do not proceed normally

The answer is: D

The incidence of group B streptococcal disease (GBS) is 1–3 cases per 1000 births. Neonates acquire the disease during birth from mothers who harbor the organism. Risk factors include prematurity, premature rupture of membranes, and group B streptococcal carriage. The Gram's stain of cerebrospinal fluid is a rapid test for GBS disease. Although sensitive, the Gram's stain requires experience to differentiate these streptococci from other Gram-positive cocci. Latex tests for GBS antigen are also available, but sensitivity in CSF is not significantly higher than the Gram's stain. GBS can be reduced by intrapartum administration of penicillin. Experimentally, GBS polysaccharide vaccines have also been used. Screening pregnant females early in pregnancy probably offers little advantage because of the possible acquisition of GBS late in the pregnancy. There has been speculation concerning the pathogenesis of GBS. These include failure to activate complement pathways and immobilization of polymorphonuclear leukocytes (PMNs) due to the inactivation of complement C5A, a potent chemoattractant. While GBS is relatively more resistant to penicillin than group A streptococci, the great majority of GBS isolates are still penicillin-susceptible. An aminoglycoside such as gentamicin may be added to GBS treatment regimens due to the relative reduced susceptibility of some strains.

22.
A man who has a penile chancre appears in a hospital's emergency service. The VDRL test is negative. Which of the following is the most appropriate course of action?
A. Send the patient home untreated
B. Repeat the VDRL test in 10 days
C. Perform dark-field microscopy for treponemes
D. Swab the chancre and culture on Thayer-Martin agar
E. Perform a Gram stain on the chancre fluid

The answer is: C

In men, the appearance of a hard chancre on the penis characteristically indicates syphilis. Even though the chancre does not appear until the infection is 2 or more weeks old, the VDRL test for syphilis still can be negative despite the presence of a chancre (the VDRL test may not become positive for 2 or 3 weeks after initial infection). However, a lesion suspected of being a primary syphilitic ulcer should be examined by dark-field microscopy, which can reveal motile treponemes.

23.
In people who have sickle cell anemia, osteomyelitis usually is associated with which of the following organisms?
A. Micrococcus
B. Escherichia
C. Pseudomonas
D. Salmonella
E. Streptococcus

The answer is: D

Many types of infection, notably respiratory tract infections and osteomyelitis, are common in people who have sickle cell anemia. For unknown reasons, Salmonella is implicated frequently in these infections. Osteomyelitis in other persons is caused most often by Staphylococcus.

24.
A hyperemic edema of the larynx and epiglottis that rapidly leads to respiratory obstruction in young children is most likely to be caused by which of the following?
A. K. pneumoniae
B. M. pneumoniae
C. Neisseria meningitidis
D. H. influenzae
E. H. hemolyticus

The answer is: D

Haemophilus influenzae is a Gram-negative bacillus. In young children, it can cause pneumonitis, sinusitis, otitis, and meningitis. Occasionally, it produces a fulminative laryngotracheitis with such severe swelling of the epiglottis that tracheostomy becomes necessary. Clinical infections with this organism after the age of 3 years are less frequent.

25.
A 70-year-old female patient was readmitted to a local hospital with fever and chills following cardiac surgery at a major teaching institution. Blood cultures were taken and a Gram-positive coccus grew from the blood cultures within 24 hours. Initial tests indicated that this isolate was resistant to penicillin.


Which of the following is the treatment of choice for the isolate?
A. Gentamicin
B. Gentamicin and ampicillin
C. Ciprofloxacin
D. Rifampin
E. No available treatment

The answer is: E

Enterococci causes a wide variety of infections ranging from less serious, for example, urinary tract infections, to very serious, such as septicemia. A Gram-positive coccus resistant to penicillin must be assumed to be enterococcus until other more definitive biochemical testing places the isolate in one of the more esoteric groups of Gram-positive cocci. Once isolated, there are a variety of tests to speciate enterococci. However, penicillin-resistant, non--lactamase-producing, vancomycin-resistant, Gram-positive cocci are most likely Enterococcus faecium. There are a variety of mechanisms for vancomycin resistance in E. faecium and they have been termed Van A, B, or C. These isolates have become one of the most feared nosocomial pathogens in the hospital environment. Unfortunately, no approved antibiotics can successfully treat vancomycin-resistant enterococci (VRE), only some experimental antibiotics such as Synercid.

26.
A 2-year-old infant is brought to the emergency room with hemolytic uremic syndrome and thrombocytopenia. Which one of the following bacteria would most likely be isolated from a stool specimen?
A. Shigella
B. Salmonella
C. Aeromonas
D. E. coli 0157/H7
E. Enterobacter

The answer is: D

Food poisoning with E. coli 0157/H7 causes hemorrhagic colitis; it is often seen after eating beef hamburgers. The same organism also causes a hemorrhagic uremic syndrome. The toxin, called Shiga-like toxin, can be demonstrated in Vero cells, but the cytotoxicity must be neutralized with specific antiserum. With the exception of sorbitol fermentation, there is nothing biochemically distinctive about these organisms.

27.
Which of the following is the most effective noninvasive test for the diagnosis of Helicobacter-associated gastric ulcers?
A. Detection of H. pylori antigen in stool
B. Growth of H. pylori from a stomach biopsy
C. Growth of H. pylori in the stool
D. IgM antibodies to H. pylori
E. Culture of stomach contents for H. pylori

The answer is: A

H. pylori antigen tests using an ELISA format and a monoclonal antibody to H. pylori are as sensitive as culture of the control portion of the stomach. Urea breath tests are also widely used. H. pylori has an active enzyme (urease) which breaks down radioactive urea. The patient releases radioactive CO2 if H. pylori are present. H. pylori antibody tests, IgG and IgA, indicate the presence of H. pylori and usually decline after effective treatment. Culture of stomach contents is insensitive and not appropriate as a diagnostic procedure for H. pylori. Direct tests such as antigen or culture of gastric mucosa are preferred because they are the most sensitive indication of a cure.

28.
A patient with symptoms of urinary tract infection had a culture taken, which grew 5 × 103 E. coli. The laboratory reported it as "insignificant." Which of the following is the most clinically appropriate action?
A. Do no further clinical workup
B. Suggest to the laboratory that low colony counts may reflect infection
C. Determine if fluorescent microscopy is available for the diagnosis of actinomycosis
D. Consider vancomycin as an alternative drug
E. Suggest a repeat antibiotic susceptibility test

The answer is: B

This question demonstrates a commonly occurring clinical infectious disease and microbiologic problem. Enterococci may be resistant to ampicillin and gentamicin. Vancomycin would be the drug of choice. However, laboratory results do not always correlate well with clinical response. The National Committee on Clinical Laboratory Standards recommends testing enterococci only for ampicillin and vancomycin.
Some symptomatic patients may have 10 leukocytes per mL of urine but relatively few bacteria. The patient is likely infected and the organisms, particularly if in pure culture, should be further processed.

29.
Which of the following is the antibiotic of choice for Lyme disease?
A. Penicillin
B. Ampicillin
C. Erythromycin
D. Vancomycin
E. Ceftriaxone

The answer is: E

There are few bacteria for which antimicrobial susceptibility is highly predictable. However, some agents are the drug of choice because of their relative effectiveness. Among the three antibiotics that have been shown to treat legionellosis effectively (erythromycin, rifampin, and minocycline), erythromycin is clearly superior, even though in vitro studies show the organism to be susceptible to other antibiotics.


Penicillin remains the drug of choice for S. pneumoniae and the group A streptococci, although a few isolates of penicillin-resistant pneumococci have been observed. Resistance among the pneumococci is either chromosomally mediated, in which case the minimal inhibitory concentrations (MICs) are relatively low, or plasmid-mediated, which results in highly resistant bacteria. The same is generally true for H. influenzae. Until the mid-1970s, virtually all isolates of H. influenzae were susceptible to ampicillin. There has been a rapidly increasing incidence of ampicillin-resistant isolates, almost 35 to 40% in some areas of the United States. Resistance is ordinarily mediated by -lactamase, although ampicillin-resistant, -lactamase-negative isolates have been seen. No resistance to penicillin has been seen in group A streptococci.


C. difficile causes toxin-mediated pseudomembranous enterocolitis as well as antibiotic-associated diarrhea. Pseudomembranous enterocolitis is normally seen during or after administration of antibiotics. One of the few agents effective against C. difficile is vancomycin. Alternatively, bacitracin can be used.


Lyme disease, caused by B. burgdorferi, has been treated with penicillin, erythromycin, and tetracycline. Treatment failures have been observed. Ceftriaxone has become the drug of choice, particularly in the advanced stages of Lyme disease.

30.
N. gonorrhoeae is a fastidious pathogen and found in sites often contaminated with normal flora. Which of the following is the best medium for isolation?
A. Sheep blood agar
B. Löffler's medium
C. Thayer-Martin agar
D. Thiosulfate citrate bile salts sucrose medium
E. Löwenstein-Jensen medium

The answer is: C

The medium of choice for the isolation of pathogenic neisseriae is Thayer-Martin (TM) agar. TM agar is both a selective and an enriched medium; it contains hemoglobin, the supplement Isovitalex, and the antibiotics vancomycin, colistin, nystatin, and trimethoprim.


V. cholerae as well as other vibrios, including V. parahaemolyticus and V. alginolyticus, are isolated best on thiosulfate citrate bile salts sucrose medium, although media such as mannitol salt agar also support the growth of vibrios. Maximal growth occurs at a pH of 8.5 to 9.5 and at 37°C incubation.

Löwenstein-Jensen slants or plates, which are composed of a nutrient base and egg yolk, are used routinely for the initial isolation of mycobacteria. Small inocula of M. tuberculosis can also be grown in oleic acid albumin media; large inocula can be cultured on simple synthetic media.


Löffler's medium, which is very rich, supports the growth of C. diphtheriae but suppresses the growth of most other nasopharyngeal microflora. C. diphtheriae colonies on this medium appear small, gray, and granular and have uneven edges.


S. aureus grows very well on sheep blood agar, which is made up of a nutrient base and 5 to 8% sheep blood; selective and differential media, such as mannitol salt agar, also are available for S. aureus.

31.
C. diphtheriae may be difficult to isolate from the nasopharynx without the use of special media. Which of the following is the medium of choice?
A. Sheep blood agar
B. Löffler's medium
C. Thayer-Martin agar
D. Thiosulfate citrate bile salts sucrose medium
E. Löwenstein-Jensen medium

The answer is: B

The medium of choice for the isolation of pathogenic neisseriae is Thayer-Martin (TM) agar. TM agar is both a selective and an enriched medium; it contains hemoglobin, the supplement Isovitalex, and the antibiotics vancomycin, colistin, nystatin, and trimethoprim.


V. cholerae as well as other vibrios, including V. parahaemolyticus and V. alginolyticus, are isolated best on thiosulfate citrate bile salts sucrose medium, although media such as mannitol salt agar also support the growth of vibrios. Maximal growth occurs at a pH of 8.5 to 9.5 and at 37°C incubation.

Löwenstein-Jensen slants or plates, which are composed of a nutrient base and egg yolk, are used routinely for the initial isolation of mycobacteria. Small inocula of M. tuberculosis can also be grown in oleic acid albumin media; large inocula can be cultured on simple synthetic media.


Löffler's medium, which is very rich, supports the growth of C. diphtheriae but suppresses the growth of most other nasopharyngeal microflora. C. diphtheriae colonies on this medium appear small, gray, and granular and have uneven edges.


S. aureus grows very well on sheep blood agar, which is made up of a nutrient base and 5 to 8% sheep blood; selective and differential media, such as mannitol salt agar, also are available for S. aureus.

32.
Which of the following bacterial transport methods is energy-independent?
A. Facilitated diffusion
B. Simple diffusion
C. Proton gradient energized active transport
D. Group translocation
E. ATP-dependent active transport

The answer is: B

Almost no important nutrients enter the bacterial cell through simple diffusion, an exception being carbon dioxide and oxygen. Some diffusion, however, is facilitated by specific protein carriers. Most transport, except simple diffusion, is energy-dependent, particularly in Gram-negative bacteria. Group translocation occurs in the absence of oxygen. For example, a simple carbohydrate such as glucose is phosphorylated enzymatically and is then transported into the cell.

33.
Iron is essential in bacterial metabolism. When bacteria invade the human host they must capture iron in order to survive. Which of the macromolecules listed below is important in bacterial iron metabolism?

A. Transferrin
B. Lactoferrin
C. Ferric oxide
D. Lipopolysaccharide (LPS)
E. Siderophores

The answer is: E

Siderophores such as aerobactin and enterobactin are chelators that trap iron Fe3+. This Fe-chelator complex is actually transported inside the cell. Transferrin and lactoferrin are iron-binding proteins found in blood and milk. Ferric or iron oxide is rust and lipopolysaccharide (LPS) is a microbial cell-wall constituent.

34.
An aliquot of Escherichia coli is treated with ethylenediaminetetraacetic acid (EDTA). The first wash is analyzed and found to contain alkaline phosphatase, DNase, and penicillinase. Which of the following anatomic areas of the cell is most likely to have been affected by the EDTA?
A. Periplasmic space
B. Mesosomal space
C. Chromosome
D. Plasma membrane
E. Slime layer

The answer is: A

The periplasm is the space between the outer membrane and plasma membrane of bacteria. The periplasmic space in Escherichia coli has been shown to contain a number of proteins, sugars, amino acids, and inorganic ions. Ethylenediaminetetraacetic acid (EDTA) is a chelating agent that disrupts the cell walls of Gram-negative bacteria.

35.
The formation of adenosine triphosphate (ATP) is essential for the maintenance of life. In mammalian systems, the number of moles of ATP formed per gram atom of oxygen consumed (the P/O ratio) is 3; in bacteria, however, the P/O ratio may be only 1 or 2. Which of the following is the primary reason for the lower P/O ratio in bacteria?
A. Absence of nicotinamide adenine dinucleotide (NAD)
B. Loss of oxidative phosphorylation coupling sites
C. Less dependence on ATP as an energy source
D. Absence of a nonphosphorylative bypass reaction
E. A less-efficient mesosome

The answer is: B

Adenosine triphosphate (ATP) is believed to be generated at three reaction points in the electron transport chain: the reductions of flavoprotein, cytochrome b, and cytochrome c. This phenomenon, demonstrated in experiments with mammalian mitochondria, can be expressed in terms of the relationship between the moles of ATP generated for each atom of oxygen consumed—the P/O ratio. In mammalian cells, the P/O ratio is 3; that is, there are three segments in the electron transfer chain in which there is a relatively large free energy drop. In bacteria, however, there appears to be only one or two of these segments. Loss of these phosphorylation sites as well as reactions that bypass these sites of ATP synthesis account for the lower P/O ratio in bacteria. Some bacteria, such as Mycobacterium phlei, have P/O ratios of 3.

36.
Reversion of Neisseria gonorrhoeae from a fimbriated (fim 1) to a nonfibriated (fim 2) state would result in which one of the following phenomena?
A. Inability of N. gonorrhoeae to colonize the mucosal epithelium
B. Reversion to a Gram-positive stain
C. Death of the organism
D. Loss of serologic specificity
E. A negative capsule strain

The answer is: A

Bacteria may shift rapidly between the fimbriated (fim +) and the nonfimbriated (fim -) states. Fimbriae function as adhesions to specific surfaces and consequently play a major role in pathogenesis. Lack of fimbriae prevents colonization of the mucosal surface by the bacterium.

37.
Certain enzymes catalyze the cross-linking of peptidoglycan, a unique constituent of bacterial cell walls. Which of the following may be a factor in antibiotic resistance, the target of which is cell-wall synthesis?
A. Bactoprenol
B. Reverse transcriptase
C. RNA polymerase
D. DNA gyrase
E. Penicillin-binding proteins (PBPs)

The answer is: E

Penicillin-binding proteins (PBPs) combine with penicillin and inhibit the final cross-linking of peptidoglycan in the cell wall. All of the other choices are involved in polymerization processes. Examples of polymerization include the cell membrane (bactoprenol) and synthesis of DNA and RNA.

38.
Early attempts at the polymerase chain reaction (PCR) used E. coli DNA polymerase. This was replaced with DNA polymerase from Thermus aquaticus ("Taq" polymerase). Which of the following is the primary advantage in using this enzyme?
A. It is cheaper than E. coli polymerase
B. Specificity is increased because nonspecific hybridization of primers does not occur
C. Use of Taq polymerase results in fewer PCR cycles
D. Use of Taq polymerase enables lower temperatures to be used
E. Upon repeated cycling, Taq polymerase becomes denatured, which causes less interference with the hybridization process

The answer is: B

DNA polymerase isolated from the hot springs thermophilic bacterium named Thermus aquaticus is essential for the polymerase chain reaction (PCR) process because of its stability at high temperatures (95°C). While the E. coli enzyme can be used, the enzyme itself becomes denatured, fewer cycles are possible, and nonspecific reactions occur because of hybridization of primers to nontarget DNA. The use of Taq polymerase allows DNA copying at 72°C rather than 37°C, which further reduces nonspecific hybridization.

39.
DNA from a host sample can be amplified by a process known as the polymerase chain reaction (PCR). Which of the following is required for PCR?
A. Knowledge of the genetic sequence to be amplified
B. An single nucleotide primer
C. An ultracentrifuge
D. A universal probe to detect the amplified product
E. A heat-sensitive DNA polymerase enzyme

The answer is: A

PCR is a widely used tool for amplification of small pieces of nucleic acid present in minute quantities. Once the sequence to be amplified is known, a specific primer is added. The temperature is alternately raised and lowered up to 45 to 50 times in the presence of a heat-resistant DNA polymerase from T. aquaticus. The amplified gene product is then detected by one of a number of techniques.

40.
Which of the following is a quinolone antibiotic with broad Gram-negative and Gram-positive activity?
A. Piperacillin
B. Cefoperazone
C. Ceftriaxone
D. Ciprofloxacin
E. Imipenem

The answer is: D

Many new antibiotics have become available during the past few years. Although expensive, these antibiotics generally have a broader spectrum of effectiveness than the ones they are intended to replace. Resistance to these newly introduced agents may be a problem that will minimize their effects on the treatment of infectious disease. While most are labeled broad spectrum, each appears to be characteristically more effective against some organisms than others.


Ceftriaxone is a new-generation cephalosporin. It is administered once a day either intravenously or intramuscularly. While ceftriaxone is used against a wide variety of Gram-negative rods, it has found special use in the treatment of Lyme disease. It is now claimed to be the most effective antibiotic for borreliosis.

41.
Which one of the following antibiotics inhibits dihydrofolate reductase?
A. Penicillin
B. Amdinocillin
C. Amphotericin
D. Chloramphenicol
E. Trimethoprim

The answer is: E

The antibiotics in these questions have significantly different modes of action. Recent evidence suggests that while penicillin inhibits the final cross-linking of the cell wall, it also binds to penicillin-binding proteins and inhibits certain key enzymes involved in cell-wall synthesis. The mechanism is complex. Amdinocillin, although classified as a penicillin, selectively binds to penicillin-binding protein-2 (PBP-2). Binding to PBP-2 results in aberrant cell-wall elongation and spherical forms, seen when E. coli, for example, is exposed to mecillinam.


Because amphotericin binds to sterols (such as cholesterol) in the cell membrane, its range of activity is predictable; that is, it is effective against microorganisms that contain sterol in the cell membrane (such as molds, yeasts, and certain amebae). These polyene antibiotics cause reorientation of sterols in the membrane, and membrane structure is altered to the extent that permeability is affected. If sterol synthesis is blocked in fungi, then amphotericin is not effective. This occurs when fungi are exposed to miconazole, another antifungal antibiotic.


Chloramphenicol is a bacteriostatic antibiotic. Its action does not kill the cell but only inhibits it. If chloramphenicol is removed from the culture, then protein synthesis is reinitiated. Bacterial ribosomes are spherical particles with a molecular weight of 3 3 106. Protein synthesis takes place on the ribosome by a complex process involving various ribosomal subunits, tRNA, and nRNA. Chloramphenicol, in contrast to the aminoglycosides and tetracycline, attaches to the 50S ribosome subunit. The enzyme peptidyl transferase, found in the 50S subunit, is inhibited. Removal of the inhibition—in this case, chloramphenicol—results in full activity of the enzyme.


Trimethoprim (TMP), a diaminopyrimidine, is a folic acid antagonist. Although TMP is commonly used in combination with sulfa drugs, its mode of action is distinct. TMP is structurally similar to the pteridine portion of dihydrofolate and prevents the conversion of folic acid to tetrahydrofolic acid by inhibition of dihydrofolate reductase. Fortunately, this enzyme in humans is relatively insensitive to TMP.

42.
Which one of the following antibiotics binds to penicillin-binding protein-2 (PBP-2)?
A. Penicillin
B. Amdinocillin
C. Amphotericin
D. Chloramphenicol
E. Trimethoprim

The answer is: B

The antibiotics in these questions have significantly different modes of action. Recent evidence suggests that while penicillin inhibits the final cross-linking of the cell wall, it also binds to penicillin-binding proteins and inhibits certain key enzymes involved in cell-wall synthesis. The mechanism is complex. Amdinocillin, although classified as a penicillin, selectively binds to penicillin-binding protein-2 (PBP-2). Binding to PBP-2 results in aberrant cell-wall elongation and spherical forms, seen when E. coli, for example, is exposed to mecillinam.


Because amphotericin binds to sterols (such as cholesterol) in the cell membrane, its range of activity is predictable; that is, it is effective against microorganisms that contain sterol in the cell membrane (such as molds, yeasts, and certain amebae). These polyene antibiotics cause reorientation of sterols in the membrane, and membrane structure is altered to the extent that permeability is affected. If sterol synthesis is blocked in fungi, then amphotericin is not effective. This occurs when fungi are exposed to miconazole, another antifungal antibiotic.


Chloramphenicol is a bacteriostatic antibiotic. Its action does not kill the cell but only inhibits it. If chloramphenicol is removed from the culture, then protein synthesis is reinitiated. Bacterial ribosomes are spherical particles with a molecular weight of 3 3 106. Protein synthesis takes place on the ribosome by a complex process involving various ribosomal subunits, tRNA, and nRNA. Chloramphenicol, in contrast to the aminoglycosides and tetracycline, attaches to the 50S ribosome subunit. The enzyme peptidyl transferase, found in the 50S subunit, is inhibited. Removal of the inhibition—in this case, chloramphenicol—results in full activity of the enzyme.


Trimethoprim (TMP), a diaminopyrimidine, is a folic acid antagonist. Although TMP is commonly used in combination with sulfa drugs, its mode of action is distinct. TMP is structurally similar to the pteridine portion of dihydrofolate and prevents the conversion of folic acid to tetrahydrofolic acid by inhibition of dihydrofolate reductase. Fortunately, this enzyme in humans is relatively insensitive to TMP.

43.
Which one of the following antibiotics inhibits the final peptide bond between d-alanine and glycine?
A. Penicillin
B. Amdinocillin
C. Amphotericin
D. Chloramphenicol
E. Trimethoprim

The answer is: A

44.
A man with chills, fever, and headache is thought to have "atypical" pneumonia. History reveals that he raises chickens and that approximately 2 weeks ago he lost a large number of them to an undiagnosed disease. Which of the following is the most likely diagnosis of this man's condition?
A. Anthrax
B. Q fever
C. Relapsing fever
D. Leptospirosis
E. Ornithosis (psittacosis)

The answer is: E

Ornithosis (psittacosis) is caused by Chlamydia psittaci. Humans usually contract the disease from infected birds kept as pets or from infected poultry, including poultry in dressing plants. Although ornithosis may be asymptomatic in humans, severe pneumonia can develop. Fortunately, the disease is cured easily with tetracycline.

45.
Which of the following mycoplasmas has been implicated as a cause of nongonococcal urethritis (NGU)?
A. Mycoplasma hominis
B. M. pneumoniae
C. M. fermentans
D. M. mycoides
E. Ureaplasma urealyticum

The answer is: E

Ureaplasma urealyticum has been associated with nongonococcal urethritis (NGU) as well as infertility. Mycoplasma pneumoniae is the etiologic agent of primary atypical pneumonia. M. hominis, although isolated from up to 30% of patients with NGU, has yet to be implicated as a cause of that disease. M. fermentans has on rare occasions been isolated from the oropharynx and genital tract. M. mycoides causes bovine pleuropneumonia.

46.
Which one of the following organisms normally inhabits the healthy human oral cavity?
A. M. hominis
B. M. orale
C. M. pneumoniae
D. M. fermentans
E. U. urealyticum

The answer is: B

Members of the mycoplasma group that are pathogenic for humans include M. pneumoniae and U. urealyticum. M. pneumoniae is best known as the causative agent of primary atypical pneumonia (PAP), which may be confused clinically with influenza or legionellosis. It also is associated with arthritis, pericarditis, aseptic meningitis, and the Guillain-Barré syndrome. M. pneumoniae can be cultivated on special media and identified by its ability to lyse erythrocytes of sheep or guinea pigs.

U. urealyticum (once called tiny, or T, strain) has been implicated in cases of nongonococcal urethritis. As the name implies, this organism is able to split urea, a fact of diagnostic significance. U. urealyticum is part of the normal flora of the genitourinary tract, particularly in women.


The only other species of Mycoplasma associated with human disease is M. hominis. A normal inhabitant of the genital tract of women, this organism has been demonstrated to produce an acute respiratory illness that is associated with sore throat and tonsillar exudate, but not with fever.


M. orale and M. salivarium are both inhabitants of the normal human oral cavity. These species are commensals and do not play a role in disease.


M. fermentans is an animal isolate.

47.
Which one of the following organisms normally inhabits the female genital tract but may cause acute respiratory illness?
A. M. hominis
B. M. orale
C. M. pneumoniae
D. M. fermentans
E. U. urealyticum

The answer is: A

48.
Which of the following is the causative agent of lymphogranuloma venereum?
A. B. (Rochalimaea) henselae
B. E. chaffeensis
C. C. trachomatis
D. R. rickettsii
E. C. burnetii

The answer is: C

Rickettsiae are small bacteria that are obligate, intracellular parasites. Most but not all rickettsiae are transmitted to humans by arthropods. Coxiella is transmitted through the respiratory tract rather than through the skin, and B. henselae, from animal scratches. Coxiella may cause chronic endocarditis that is not very responsive to either antimicrobial therapy or valve replacement. B. henselae is a fastidious Gram-negative rod that causes bacillary angiomatosis, a disease that forms dermal or subcutaneous nodules. The role of B. henselae in cat-scratch disease has recently been recognized. Molecular taxonomic studies have indicated that the causative organism is more closely related to Bartonella than Rochalimaea, hence the name change.


Ehrlichia is an obligate, intracellular parasite that resembles rickettsia. E. chaffeensis has been linked to human ehrlichiosis, although this infection is primarily seen in animals. The majority of patients with this disease report exposure to ticks. It is thought that I. scapularis carries Ehrlichia, although the Lone Star tick, A. americanum, may also transmit the disease.


Chlamydiae are Gram-negative bacteria that are obligate, intracellular parasites. They are divided into three species: C. trachomatis, C. pneumoniae, and C. psittaci. Chlamydiae have a unique developmental cycle. The infectious particle is the elementary body. Once inside the cell, the elementary body undergoes reorganization to form a reticulate body. After several replications, the reticulate bodies differentiate into elementary bodies, are released from the host cell, and become available to infect other cells. Three of the 15 serovars of C. trachomatis (L1, L2, L3) are known to cause lymphogranuloma venereum (LGV), a sexually transmitted disease. C. trachomatis is a leading cause of sexually transmitted disease in the United States. It is insidious because so many early infections are asymptomatic, particularly in women.


Rocky Mountain spotted fever is a spotted fever caused by R. rickettsii and is characterized by acute onset of fever, severe headache, and myalgias. The rash occurs 2 to 6 days later first in the hands and feet and then moves to the trunk. Diagnosis must be made on clinical presentations, and therapy instituted immediately. Laboratory diagnosis is made on a rising antibody titer (delayed). Untreated disease can be fatal.

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