USMLE USMLE Certifications USMLE-STEP-1 Questions & Answers

• Question 371:

  A 35-year-old male with end-stage renal disease must undergo a kidney transplant. Which of the following is true of immunologic suppression for transplantation?

  A. It can occur by antilymphocyte globulin.

  B. It cannot be achieved by cyclosporine administration.

  C. It cannot occur by lymphoid irradiation.

  D. It is facilitated by gamma interferon administration.

  E. It is not likely to respond to steroid administration.

  Correct Answer: A

  Section: Microbiology/Immunology There are various ways by which immunologic suppression for transplanted tissue can occur. One of these ways is administration of antilymphocyte globulin. Other means used to obtain immunologic suppression for transplantation include destruction of B and T lymphocytes by irradiation (choice C). Cyclosporin is also used to achieve immunologic suppression for transplantation. It is thought to inhibit interleukin-2, which drives antigen-activated cells to proliferate (choice B). Steroid administration is an effective means to suppress the immune system (choice E). Suppression of transplanted tissue cannot be obtained by interferon administration, because interferon inhibits translation of viral mRNA (choice D).

• Question 372:

  Oseltamivir is used for the treatment of infections by a virus with a segmented negative strand genome. Which of the following would respond to this treatment?

  A. EBV

  B. herpes simplex virus

  C. influenza virus

  D. rabies virus

  E. rhinovirus

  Correct Answer: C

  Section: Microbiology/Immunology The NA inhibitors zanamivir and oseltamivir (tamiflu) were approved in 1999 for the treatment of influenza Aand influenza B. Amantadine (generic name) or Symmetrel (trade name) inhibits an early event in the multiplication cycle of influenza virus, as well as arenaviruses. It blocks the uncoating process. Mutations in the M protein genes result in the development of drug-resistant mutants. The drug is not used extensively in the United States because it seems impractical to control this type of infectious disease that is not ordinarily fatal. To protect individuals at high risk, and in those in whom the infection is potentially dangerous, the choice is between this drug and the influenza vaccine. In most cases, the vaccine is usually preferred. Acyclovir is used to shorten the duration of herpes simplex virus episodes and also to limit the duration of viral shedding (choice B). There are no antiviral drugs for rabies (choice D). Acyclovir has slight antiviral action for EBV (choice A). There is no antiviral agent against rhinovirus (choice E).

• Question 373:

  A 21-year-old man has been skin tested and had a positive test to purified protein derivative (PPD). This indicates that he has which of the following?

  A. active tuberculosis

  B. proper B-cell function

  C. provided sound information on his health status

  D. received BCG vaccine

  E. received middlebrook 7H10/7H11 vaccine

  Correct Answer: D

  Section: Microbiology/Immunology A positive PPD skin test indicates that the individual has experienced mycobacterial infection at some time. Apositive PPD skin test is usually obtained in individuals who have been vaccinated with the attenuated strain of M. bovis produced by Calmette-Guerin, who named the vaccine bacillus-Calmette- Guerin (BCG). However, as has been indicated above, any individual who had contact with M. tuberculosis usually gives a positive PPD test. The PPD skin test does not necessarily indicate current disease (choice A) or give any information on the health status of the individual (choice C). The PPD skin test may be used to assess proper T-cell function, but not B-cell function (choice B). Middlebrook 7H10/7H11 (choice E) refers to media, not vaccines.

• Question 374:

  Which of the following colonizes the nasopharynx and produces toxins that kill ciliated epithelial cells, causes edema locally, and induces lymphocytosis systemically?

  A. B. pertussis

  B. Hemophilus influenzae type b

  C. L. pneumophila

  D. N. meningitides

  E. P. aeruginosa

  Correct Answer: A

  Section: Microbiology/Immunology

  B. pertussis (choice A) is a highly communicable small gram-negative coccobacillus that causes whooping cough (pertussis). It is isolated on Bordet-Gengou medium. B. pertussis produces a number of factors involved in the pathogenesis of the disease. Ahemeagglutinin mediates attachment of the bacteria to ciliated epithelial cells. The pertussis toxin promotes lymphocytosis, histamine sensitization, and a mechanism of action similar to cholera toxin. Atracheal exotoxin inhibits DNA synthesis in ciliated cells. B pertussis survives only briefly outside the human host. During the catarrhal stage, large numbers of infectious bacteria are released in respiratory secretions. It is during the paroxysmal stage that the cough develops its explosive character and the "whoop" upon inhalation. Infants are especially at risk with persistent coughing in older children and even adults. An excellent vaccine is available to control this disease. The other choices (B, C, D, and E) are also upper respiratory pathogens capable of causing serious disease episodes. None of them produce the toxic products that are made by B. pertussis that are directly responsible for the action on cells and symptoms observed in whooping cough.

• Question 375:

  A child has conjunctivitis and sore throat. The clinical laboratory has isolated a nonenveloped, icosahedral DNA virus shown in below figure. Which of the following is likely the infectious agent?

  

  A. adenovirus

  B. Hepadnavirus

  C. herpesvirus

  D. parvovirus

  E. poxvirus

  Correct Answer: A

  Section: Microbiology/Immunology Adenoviruses are the only ones that produce spikes extending from each one of the vertices of the capsid. These microbes are DNA, nonenveloped, icosahedral viruses. Adenoviruses causepper respiratory infections associated with coryza, sore throat, fever, and conjunctivitis

• Question 376:

  A 30-year-old man presents symptoms and signs consistent with primary syphilis. Which of the following pairs of tests are more likely to provide a definitive confirmation of this diagnosis?

  A. dark-field microscopy followed by Venereal Disease Research Lab (VDRL) test

  B. examination of sample taken from a chancre by dark-field microscopy followed by fluorescent antibody (FA) technique

  C. silver staining of sample taken from the patient followed by VDRL

  D. VDRL followed by FA

  E. VDRL followed by Fluorescent Treponemal Antibody Absorption (FTA-Abs) test

  Correct Answer: E

  Section: Microbiology/Immunology The fluorescent treponemal antibody absorption test (FTA-ABS) is the confirmatory test of choice for syphilis. This is a standard indirect fluorescent antibody test. It uses killed T. pallidum as a specific antigen, the patient's serum which has been absorbed with an extract of nonpathogenic spirochetes to remove cross-reacting antibodies, and the fluorescein isothiocynate-labeled antihuman antibody versus the regular florescent antibody (FA) alone. This test is positive earlier in primary syphilis, and remains positive in more patients with latent syphilis than the VDRL, RPR, or other tests for syphilis. The VDRL and the RPR tests are known as the nontreponemal tests for syphilis, and they are used for screening large groups of people. They measure nonspecific IgG and IgM called reagins, which are directed against a cardiolipin extracted from normal mammalian tissues. The silver stain (choice C) is used to detect P. jroveci cysts in specimens from AIDS patients. The correct procedure for primary syphilis diagnosis confirmation would be to screen a serum with a general screening test (VDRL) and onfirm a positive VDRL with a more specific FTA-Abs test specific for antisyphilis antibodies (choice E).

• Question 377:

  A Gram stain of pus from the oral abscess of a 62-year-old man with periodontitis shows that the patient's infection is polymicrobic. Which of the following is the most likely source of this man's infection?

  A. A. israelii following tooth extraction

  B. autoinfection with oral anaerobes

  C. autoinfection with oral Staphylococcus spp.

  D. periodontitis is not an infectious process

  E. P. aeruginosa following primary viral infection

  Correct Answer: B

  Section: Microbiology/Immunology Medically important infections due to anerobic bacteria are common. The infections are often polymicrobial, anerobes being mixed with other anerobes, facultative anerobes, and aerobes. Anerobic bacteria are found throughout the body's normal or usual flora. They comprise part of the bacteria on the skin, on mucosal surfaces, in the mouth, and in the intestinal tract. Infection occurs when anerobes contaminate body sites that are normally sterile. Such conditions may also arise with the use of broad spectrum antimicrobials. In the oral cavity, anerobes are present in small numbers. Bacteroides, Fusobacterium, Prevotella, and Porphyromonas make up the most common genera. The main etiological agents of periodontitis (soft oral tissue infections) and endodontic (root canal nfections) belong to those four genera. Therefore, choice B represents a situation where low numbers of endogenous, normal anerobic bacteria were introduced into a lesion area and made the infection orse. Such oral infections will respond to dental manipulations and antibiotics. Periodontitis (choice D) is definitely an infectious process. Another important factor is the polymicrobial etiology from endogenous sources can be found in the lesions. Choices A (Actinomyces), C (Staphylococcus), and E (Pseudomonas) ould be incorrect since they present a single genus in each case for etiology.

• Question 378:

  A 53-year-old alcoholic man had been experiencing acute episodes of nausea and eventually jaundice. Avirus was detected from him, which had a circular molecule of DNA having the structure diagrammed in below figure. Which virus listed below has this type of genome?

  

  A. EBV

  B. hepatitis A virus

  C. hepatitis B virus

  D. JC virus

  E. papillomavirus

  Correct Answer: C

  Section: Microbiology/Immunology The genome of hepatitis B virus is composed of a circular, double-stranded DNA. It has a negative strand of 3200 nucleotides and another positive, incomplete strand of 17002600 nucleotides. Papilloma (choice E) and polyomaviruses belong to the family of Papoviridae, which lacks envelopes and have a double-stranded, circular DNA genome. Both strands are complete and thus differ from the genome of hepatitis B virus that has one incomplete positive strand. Hepatitis Avirus (choice B), the cause of infectious hepatitis, has a linear single-stranded RNA genome, and as such it is a member of the enteroviruses. EBV (choice A) is a member of the herpesviruses and as such has a double-stranded, linear DNA genome. EBV has been associated with infectious mononucleosis, Burkitt lymphoma, and nasopharyngeal carcinoma. JC virus (choice D) is a member of the polyomaviruses.

• Question 379:

  Asuburban housewife was pricked by a thorn while pruning her roses. Four days later, a pustule that changed to an ulcer developed in the lesion area. Then three nodules formed along the local lymphatic drainage. The most likely agent is which of the following?

  A. A. fumigates

  B. C. albicans

  C. Cryptococcus neoformans

  D. Sporothrix schenckii

  E. Trichophyton rubrum

  Correct Answer: D

  Section: Microbiology/Immunology

  S. schenckii is found on thorns, and it is introduced into the skin of extremities through trauma. A regional lesion begins as a pustule, abscess, or ulcer, and then nodules and abscesses are formed along the lymphatics. The history and the symptoms described in this patient are consonant with a diagnosis of sporotrichosis. A. fumigatus (choice A) and C. albicans (choice B) are associated with deep opportunistic infections in immunocompromised patients such as AIDS patients. Aspergillosis is basically a pulmonary infection. Candidiasis can be associated with pathological conditions of the mucous membranes of the respiratory, genital, and gastrointestinal tract, where it is found as a normal inhabitant. C. neoformans (choice C) is the cause of meningitis. T. rubrum (choice E) is the cause of dermatophytosis ringworm of skin, scalp, and especially nails. The nails thicken and are discolored.

• Question 380:

  A 58-year-old nurse has a herpetic infection that requires administration of acyclovir. Which one of the following is the known mechanism of action of acyclovir?

  A. degradation of viral RNA

  B. inhibition of reactivation of virus in trigeminal root ganglia

  C. inhibition of viral RNA synthesis

  D. killing of actively dividing cells in which virus replicates

  E. termination of DNA replication of herpes virus

  Correct Answer: E

  Section: Microbiology/Immunology Acyclovir is a guanosine analogue which inhibits viral DNA synthesis. It is activated in cells by the virus-encoded thymidine kinase, which phosphorylates this drug. Only viruses that produce thymidine kinase, such as herpes simplex 1 and 2 or the varicella-zoster virus are susceptible to acyclovir. This antiviral agent has not been shown to inhibit reactivation of herpes virus in trigeminal ganglia (choice B), to kill actively dividing cells in which virus replicates (choice D), to degrade (choice A), or to inhibit viral RNA synthesis (choice C).