USMLE USMLE Certifications USMLE-STEP-1 Questions & Answers

• Question 401:

  Clinical laboratory results indicate that an AIDS patient in a county hospital in San Antonio is infected with an acid-fast bacillus. Which of the following bacteria is this organism most likely to be?

  A. M. avium-intracellulare

  B. M. fortuitum-chelonei

  C. M. marinum

  D. M. scrofulaceum

  E. M. tuberculosis

  Correct Answer: A

  Section: Microbiology/Immunology

  M. avium-intracelluaire (choice A) is often called the MAC or MAI (M. avium-intracellulaire) complex. They are ubiquitous in the environment (water, soil, food, animals). MAC organisms seldom cause disease in immunocompetent humans but are efficient opportunists in those immunocompromised, especially in AIDS patients in the United States. In fact, it is one of the most common opportunistic infections of bacterial origin in that situation. Environmental exposure can lead to MAC colonization of either the respiratory or GI tract. A transient bacteremia is usually followed by tissue invasion. Any organ may be affected. A wide variety of clinical presentations may occur, but patients often present with nonspecific symptoms of fever, night sweats, abdominal pain, and diarrhea and weight loss. Diagnosis is made by culturing the organism. Treatment may involve multiple drugs and last from 6 months to 1 year or even for life. M. fortuitum-cheloni (choice B) are soil/water saprophytes and may cause superficial or systemic disease only rarely. M. marinum (choice C) occurs in water and may cause skin lesions in humans. Infections are relatively rare.M. scrofulaceum (choice D) is also found in water and may be a rare saprophyte in adults with chronic lung disease. M. tuberculosis (choice E) causes tuberculosis in humans and is a very important human pathogen. Host to host transmission is the usual method observed.

• Question 402:

  A Gram-stain smear of spinal fluid from a 2- year-old child reveals short gram-negative rods. It grows on enriched chocolate agar, but does not grow on blood agar, except adjacent to a streak of staphylococci. The organism most probably is which of the following?

  A. H. influenza

  B. N. gonorrhoeae

  C. N. meningitides

  D. L. monocytogenes

  E. S. pneumoniae

  Correct Answer: A

  Section: Microbiology/Immunology The organisms of the genus Haemophilus are small, gram-negative, nonmotile, nonspore-forming bacilli with complex growth requirements. H. influenzae requires a heatstable factor found in blood (X factor), which can be replaced by hematin, and nicotinamide adenine dinucleotide (V factor), which can be added to the medium as a supplement, or can be supplied by other microorganisms, such as staphylococci (satellite phenomenon). N. meningitides (choice C), N. gonorrhoeae (choice D), S. pneumoniae (choice E), and L. monocytogenes (choice B) are able to synthesize hematin and nicotinamide adenine dinucleotide, and thus they will grow in culture media which do not contain these nutrients. They also do not need to rely on the production of hematin and nicotinamide adenine dinucleotide by S. aureus to grow. In contrast to H. influenzae, S. pneumonia is a gram-positive, lancet- shaped diplococcus, not a gram-negative bacterium.

  L. monocytogenes is a gram-positive rod.

• Question 403:

  In a quantitative serological test, the results shown below were obtained (+ = antibody detected, 0 = no antibody detected). Which of the following courses of action should be taken?

  

  A. choose another test mechanism to measure the antibody

  B. perform testing on additional dilutions until an end point is reached

  C. repeat the test because this pattern is impossible

  D. report "negative" for antibody E) report "positive" for antibody

  Correct Answer: B

  Section: Microbiology/Immunology Quantitative serological tests are being replaced by newer testing procedures, such as ELISA, which is not quantitative. Understanding how quantitative data is used diagnostically is still important clinically. For best results for interpretation, an acute and convalescent serum from thepatient should be tested at the same time. If a 4-fold (1:2 dilutions) or 100-fold (1:10 dilutions) increase in Ab titer is found, that would be a positive diagnostic result. Often, as in this case, only a single serum sample was tested. Positive results through the highest dilution of serum could represent a very high Ab level or some problem with the test procedure, giving a false-positive interpretation. The most effective way to solve this problem is to dilute the serum further and repeat the test (choice B). Once an end-point is reached, this information may confirm or deny a possible diagnosis. An alternative test mechanism (choice A) may be available, but one must remember not to use results from two test mechanisms as a direct comparison for diagnostic purposes. This pattern is not uncommon (choice C) because high antibody titers may exist in some infections (EBV, for example). Negative (choice D) or positive (choice E) would be inappropriate reports without repeating the test as described above.

• Question 404:

  A 30-year-old previously healthy patient has all the gastrointestinal symptoms of infection with hepatitis A virus (HAV), yet all the tests for HAV-IgG and HAV-IgM are negative. Which of the following is the most likely cause of this infection?

  A. hepatitis B virus

  B. hepatitis C virus

  C. hepatitis D virus

  D. hepatitis E virus

  E. rotavirus

  Correct Answer: D

  Section: Microbiology/Immunology Hepatitis E virus (choice D) has been tentatively placed in the calicivirus grouping. It has been found to be the cause of another form of hepatitis and is spread by the oral-fecal route, resembling hepatitis A epidemiology. This virus, like HAV, appears to cause only acute disease and is especially dangerous to pregnant women. This cohort experienced a 20% mortality rate in an Indian outbreak in 1955. No specific diagnostic tests are currently available for HEV diagnosis. Hepatitis B (HBV--choice A) and hepatitis C (HCV-- choice B) are usually spread by means of blood and excellent diagnostic tests exist for these to diagnose specifically or rule out these agents. Hepatitis D virus (HDV--choice C) contains delta-Ag surrounded by HBV surface antigen. HDV must exist in a coinfection with HBV and may present as an acute or chronic infection. Diagnostic tests for HDV also exist. Rotaviruses (choice E) are double-stranded RNA viruses that are usually found in pediatric infections.

• Question 405:

  After isolating a pathogen from a patient, you monitor its growth in rich medium. According to below figure the rate of growth of the bacterial culture for which it was obtained reaches its maximum rate of growth between which of the following time periods?

  

  A. noon and 1:00 p.m.

  B. 2:00 p.m. and 3:00 p.m.

  C. 3:00 p.m. and 4:00 p.m.

  D. 3:00 p.m. and 5:00 p.m.

  E. 4:00 p.m. and 5:00 p.m.

  Correct Answer: B

  Section: Microbiology/Immunology By definition, the rate of growth of bacteria represents the change in the bacterial cell numbers over the change in time. From the choices given, the maximum rate of growth occurs between 2 p.m. and 3 p.m., where within 1 hour the number of bacteria has increased approximately threefold. Between noon and 1 p.m., 34 p.m., and 35 p.m., there is no increase in the number of cells and the rate of growth is zero.

  

• Question 406:

  A 64-year-old alcoholic man has fever, chills, cough, and pleuritic pain. His sputum is a dark brown color, and upon cultivation on blood agar produces alpha hemolytic colonies, composed of gram- positive, optochin-positive cocci. The microbes are which of the following?

  A. E. faecalis

  B. N. meningitides

  C. S. aureus

  D. S. pneumonia

  E. S. pyogenes

  Correct Answer: D

  Section: Microbiology/Immunology The main symptoms of pneumonia caused by S. pneumoniae are fever, chills, and cough that produce a dark brown sputum. Alcoholism predisposes an individual to pneumonia because it reduces phagocytic activity, and promotes aspiration of microbes. In addition to S. pneumoniae (choice D), S. pyogenes (choice E), S. aureus (choice C), and N. meningitidis (choice B) can cause pneumonia. However, these other microbes do not produce alpha hemolysis and are not sensitive to optochin.

• Question 407:

  Which of the following substances is most likely to prevent the attachment of the mold allergen to the sensitized mast cells in this patient?

  A. complement

  B. corticosteroids

  C. cromolyn sodium

  D. epinephrine

  E. IgG anti-mold

  Correct Answer: E

  Section: Microbiology/Immunology The IgG anti-mold-blocking antibody will combine with the mold allergen and will not permit the mold allergen to reach the IgE antimold on the surface of the mast cells, thus inhibiting the allergic reaction. Corticosteroids suppress inflammation, but will not prevent the attachment of the mold allergen to the sensitized mast cells by the patient (choice B). Complement is not fixed by IgE allergen complexes and plays no role in the attachment of mold allergen to sensitized mast cells (choice A). Epinephrine reverses constriction of bronchioles and bronchi, and also has no effect on the attachment of mold allergen to sensitized mast cells (choice D). Cromolyn sodium stabilizes the membrane of mast cells and thus prevents release of histamine from mast cells (choice C).

• Question 408:

  A 34-year-old female patient is sneezing and has a runny nose and watery eyes every summer. Her physician is convinced that the patient is suffering from an allergy and performs some skin tests. The results of these tests are shown below:

  

  Which of the following test allergens is most likely to be inducing the patient's symptoms?

  A. cat dander

  B. house dust

  C. Kentucky blue grass

  D. mold

  E. pollen

  Correct Answer: C

  Section: Microbiology/Immunology Ms Y is suffering from allergic rhinitis. The diagnosis of this allergy is made on the basis of clinical symptoms and the performance of a skin test, as well as a radioallergosorbent test (RAST). In a skin test, a battery of potential allergens is injected separately subcutaneously, and the area of the wheal and flare reaction is measured. The allergen which induces the greatest wheal and flare reaction when compared to solvent is usually the one which causes the allergy. In this case injection of Kentucky blue grass yielded the biggest wheal and flare reaction, and thus this is the allergen which caused rhinitis in the patient. Allergens such as mold (choice D), cat dander (choice A), house dust (choice B), and pollen (choice E) gave wheal and flare reactions which were either below or slightly above the skin test values of the allergen solvent. Therefore, they are not likely to be causing allergy in the patient.

• Question 409:

  A female patient presents with swollen inguinal lymph nodes and painful lesions on the genitalia and perianal region. Which of the organisms below is the most likely causative agent for this infection?

  A. Group B streptococci

  B. Haemophilus aegypticus

  C. H. ducreyi

  D. Moraxella (Branhamella) catarrhalis

  E. N. gonorrhoeae

  Correct Answer: C

  Section: Microbiology/Immunology

  H. ducreyi (choice C) causes chancroid (soft chancre), a sexually transmitted disease. Chancroid consists of a ragged ulcer on the genetalia, with marked swelling and tenderness. The regional lymph nodes are usually enlarged and painful. Differential diagnosis must consider syphilis, HSV infection, or lymphogranuloma venereum. The organisms can usually be obtained from the lesions and often are part of a mixed infection. Scrapings from the lesion base are inoculated on to chocolate agar with X factor and vancomycin. No permanent immunity develops from chancroid infection, but the organism is treatable with ceftriaxone, TMS, or erythromycin. Group B streptococci (choice A) are usually a component of normal vaginal flora, but do not tend to be pathogenic to the host. Group B streptococci are most significant in new-born infections. H. egypticus (choice B) is most often found to be associated with conjunctivitis and Brazilian purpuric fever in children. M. catarrhailis (choice D) is a usual oral cavity organism and is considered to be an opportunist, although it has not been associated with genital infections. N. gonorrhoeae (choice E) will usually present as a mucus membrane infection with a discharge and not present as genital lesions.

• Question 410:

  Which of the following immunological assays employs separation of proteins by electrophoresis in polyacrylamide gels and subsequent transfer to nitrocellulose, followed by a reaction with antibodies?

  A. complement fixation

  B. enzyme-linked immunosorbent assay (ELISA)

  C. immunoelectrophoresis

  D. indirect fluorescent antibody test

  E. Western blot

  Correct Answer: E

  Section: Microbiology/Immunology In Western blots proteins are separated on the basis of molecular weight in polyacrylamide gels by electrophoresis. Then, they are transferred electrophoretically to nitrocellulose membranes and reacted with antibodies and substrate for detection. The Western blot assay is used as a confirmatory test in the determination of the HIV status of an individual. Electrophoretic transfer of proteins or antigens to nitrocellulose membranes followed by antibody reaction is not a part of complement fixation (choice A), indirect fluorescent antibody test (choice D), ELISA (choice B), or immunoelectrophoresis (choice C).