USMLE USMLE Certifications USMLE-STEP-1 Questions & Answers

• Question 421:

  A 45-year-old male experiences memory lapses, loss of reasoning ability, ataxia, and myoclonic jerking. His condition steadily deteriorates and finally progresses to stupor, coma, and death. The physician notes that 34 years ago, the patient was given injections of growth hormone prepared from human pituitary glands. Histological sections of brain show widespread neuron loss, vacuolation, and inflammation. Which of the following is the most likely diagnosis?

  A. Creutzfeldt-Jacob disease (CJD)

  B. kuru

  C. meningitis due to herpes simplex virus

  D. progressive multifocal leukoencephalopathy (PML)

  E. subacute sclerosing panencephalitis (SSPE)

  Correct Answer: A

  Section: Microbiology/Immunology CJD (choice A) is caused by a prion, a protein agent with no detectable nucleic acid for a genome. It is considered to be one of a group of degenerative central nervous diseases called transmissible spongiform encephalopathies. Its incubation period most often lasts for years. CJD in humans begins gradually, with progressive dementia, ataxia, twitching of muscles (myoclous), and leading to death within a year. While cases are rare, there may be some type of familial association. Kuru (choice B) is similar to CJD but was confined to New Guinea. Once consumption of dead relatives was outlawed, the disease disappeared. Herpes simplex meningitis (choice C) results in a fairly rapid onset of symptoms caused by destructionof brain tissue, followed by a mortality rate of 7580%. Acyclovir treatment is usually beneficial. PML (choice D) is caused by JC Virus (JCV), a polyomavirus. It is considered a slow virus disease. Several polyomaviruses are important DNA tumor viruses. PML is seen in about 5% of patients with AIDS. Clinically, demyelization of the CNS results in oligodendrocyte infection with polyomaviruses. SSPE (choice E) is another slow virus disease, caused by a defective variant of the measles virus. This slowly progressive demyelization of the CNS results in death.

• Question 422:

  A rabbit is repeatedly injected with a peptide hapten. Two weeks later, its serum is subjected to a gel diffusion assay with the hapten and a carrier protein. Which of the following would be the expected result of the gel diffusion assay?

  A. A line of identity between serum and both the carrier and the hapten will be present.

  B. A line of identity between the serum and carrier protein will be detected.

  C. A line of nonidentity between serum, carrier, and hapten will be detected.

  D. A line of partial identity between serum, carrier, and hapten will be detected.

  E. No precipitin line will be present.

  Correct Answer: E

  Section: Microbiology/Immunology By definition, a hapten is a substance of low molecular weight that by itself does not elicit the formation of antibodies. However, when attached to a carrier protein, antibody production becomes possible. The haptencarrier approach has been employed to produce antibodies against penicillin, steroids, nucleotides, lipids, and even 2,4-dinitrophenol. Since the rabbit has been repeatedly injected with the hapten only, the serum cannot be expected to have antibodies against the hapten. It is clear that when the serum is subjected to a gel diffusion assay with the hapten and a carrier protein not used to complex the hapten during immunization, no antigen-antibody precipitin lines of either identity, partial identity, or nonidentity can be expected to form (choices A, B, C, and D).

• Question 423:

  A 3-year-old child arrives at the hospital in severe respiratory distress. Periorbital edema is also noted. Blood analysis reveals decreased levels of C4 and decreased levels of C1 inhibitor. This complement deficiency is likely to lead to which of the following conditions?

  A. angioedema

  B. an increased susceptibility to pyogenic infections

  C. bacteremia

  D. decreased production of anaphylatoxins

  E. enhancement of antibody production

  Correct Answer: A

  Section: Microbiology/Immunology Deficiency in the inhibitor of the first component of complement (C1) is associated with angioedema, because this condition leads to the production of anaphylatoxins C3a, C4a, and C5a. These anaphylatoxins act on mast cells, which release large amounts of histamine. Production of histamine increases capillary permeability resulting in edema. Enhanced susceptibility to pyogenic infections has been attributed to C3 and C5C8 components of complement. Bacteremia is a term used to indicate the presence of bacteria in the blood. Thus it has nothing to do with the first component of complement (choice C). Inherited deficiencies of C3 and C5C8 have been associated with increased susceptibility to pyogenic infections, but not the first component of complement (choice B). Complement can act synergistically with antibody to modify its action, but not its production (choice E). The conversions of the third (C3) and fifth (C5) components of complementby the C3 and C5 convertases to C3a and C5a anaphylatoxins lead to the increased production of anaphylatoxins (choice D) seen with C1 inhibitor deficiency.

• Question 424:

  Aurease-positive, spiral-shaped gram-negative rod is found in gastric washings from a 29-year-old stressed-out medical student with gastritis. Which of the following is most likely to be this organism?

  A. Campylobacter jejuni

  B. Helicobacter pylori

  C. Proteus mirabilis

  D. Providencia rettgeri

  E. Vibrio cholerae

  Correct Answer: B

  Section: Microbiology/Immunology

  H. pylori is a spiral-shaped gram-negative motile rod that is associated with antral gastritis and duodenal (peptic) ulcer disease. It is oxidase-positive, catalase-positive, and a strong producer of urease. Antimicrobial therapy that clears H. pylori results in improvement of gastritis and duodenal ulcer disease. Hence, choice B is the correct answer. C. jejuni (choice A) infections are widespread and usually cause abdominal pain and profuse bloody diarrhea, not gastritis and ulcer manifestations. Ureasepositive P. mirabilis (choice C) and other Proteus species produce infections in humans when the bacteria leave the intestinal tract, being very significant in urinary tract and bacteremia infections. P. retteri (choice D) is a member of the normal intestinal flora, often resistant to antimicrobial therapy, and is seen in urinary tract infections. V. cholerae is a comma-shaped curved rod and is widely distributed in marine and surface waters in nature. It is oxidase positive and many species are salt tolerant. V. cholerae (choice E) produces a heat-labile enterotoxin whose action increases cAMP in intestinal cells, resulting in prolonged hypersecretion of water and electrolytes.

• Question 425:

  After an abdominal surgery for removal of ovarian cysts, a 56-year-old female has had low-grade fever for the past 2 weeks. She has a history of rheumatic fever as a child. Three of the blood cultures grew gram-positive cocci. Which of the following is the most likely etiologic agent?

  A. Group A streptococci

  B. group B streptococci

  C. group C streptococci

  D. group D streptococci

  E. S. viridans

  Correct Answer: D

  Section: Microbiology/Immunology There are at least 12 species of enterococci, and all are classified as group D streptococci (choice D). E. faecalis is the most common and causes 8590% of enterococcal infections. As long as enterococci remain in the intestinal tract, they are seldom involved in disease. Yet, they are extremely efficient opportunists. The enterococci are among the most frequent causes of nosocomial infections, especially in intensive care units. They are commonly found in complications of intra-abdominal surgery, often as part of a mixed culture of microorganisms. Antibiotic therapy (cephalosporins, etc) is often a selective feature since enterococci are resistant to many of the antimicrobials in common use. Group Astreptococci (choice A) would be represented by S. pyogenes and is the group which causes about 95% of human infections (skin, septicemia, organs, etc), but would seldom be found in intra-abdominal complications. Streptococci B (choice B) are most often found in infections of infants since type B streptococci are found routinely in the vaginal flora. Group C streptococci (choice C) usually occur in the nasopharynx and may cause sinus, blood, or heat infections. S. viridans (choice E) are usually found in the oral cavity and may be opportunists involving blood and heart infections.

• Question 426:

  Which of the following will be observed in a person who is deficient in interleukin-1 synthesis?

  A. difficulty with immunoglobulin isotypic switching

  B. dependence on endotoxin for the induction of febrile responses

  C. effective mediation of acute phase responses

  D. effective regulation of vascular shock

  E. tendency to develop intense inflammatory reactions

  Correct Answer: B

  Section: Microbiology/Immunology Interleukin-1 (IL-1) is a protein generated primarily by macrophages. The cytokine is called endogenous pyrogen because it acts on the hypothalamus to produce fever during infections and inflammatory conditions. Thus individuals who are unable to produce IL-1 most likely will have to depend on endotoxin, the exogenous pyrogen, to induce febrile reactions. Mediation of immunoglobulin isotypic switching (choice A) requires IL-4 and IL-5. Acute-phase responses (choice C) and shock (choice D) are mediated by IL-6 and TNFalpha, TNF-alpha. Intense inflammatory reactions are produced by IL-1 (choice E).

• Question 427:

  Antibodies against acetylcholine neural receptors were measured in a 35-year-old woman and are thought to be involved in the pathogenesis of which of the following?

  A. acute idiopathic polyneuritis

  B. Guillain-Barr syndrome

  C. multiple sclerosis

  D. myasthenia gravis

  E. postpericardiotomy syndrome

  Correct Answer: D

  Section: Microbiology/Immunology Anti-acetylcholine receptor antibodies are found in more than 90% of myasthenia gravis patients. If the clinical symptoms are suggestive of myasthenia gravis, this finding alone is often considered diagnostic. Multiple sclerosis patients tend to have high levels of measles virus antibodies in their spinal fluid. However, the role of this agent in the disease is undetermined (choice C). Guillain-Barr syndrome (also called acute idiopathic polyneuritis) is a demyelinating disease of peripheral nerves. It commonly occurs after a viral infection or an injection, such as influenza immunization. The disease seems to be caused by a T-cell response to nervous tissue (choices A and B). Postpericardiotomy syndrome is a term used to describe a disorder following surgery of the pericardium to remove cysts or tumors or to correct a malformation (choice E).

• Question 428:

  During a pelvic examination, a woman is found to have painful vesicular lesions on the vagina. The patient states that she had similar lesions 12 months previously which lasted for 2 weeks. Which of the following is the most likely causative agent?

  A. cytomegalovirus

  B. herpes simplex virus

  C. papillomavirus

  D. rubella virus

  E. varicella virus

  Correct Answer: B

  Section: Microbiology/Immunology Recurrent vesiculating lesions in the genital region suggest human herpesvirus type II, although type I is seen in some cases. Choice B would be the correct answer. Cytomegalovirus (choice A) and rubella virus (choice D) will commonly form latent infections but would not appear as vesicular lesions in the genital area if reactivated. Rubella may occur as a macular rash on the skin, but only in the initial infection episode. Papillomaviruses (choice C) are highly tropic for epithelial cells of the skin and mucus membranes. Papilloma lesions (warts) are benign except for certain strains and are never presented as vesicular. Warts contain lesion parts known as the horny layer and the granular layer. Varicella (choice E) or chickenpox will usually have all stages of a lesion (macule, papule, vesicle, and pustule) present and would be skin lesions, not in the genital mucus membrane area of the body.

• Question 429:

  An 8-month old boy presents to the emergency room in respiratory distress from a recurrent upper respiratory tract bacterial infection. Labs reveal low levels of IgA, IgD, IgE, IgG, and IgM. Suspecting animmune deficiency disorder, genetic testing reveals a defect in a tyrosine kinase gene. Which of the following would be seen in patients with this immune deficiency disorder?

  A. particular susceptibility to viral and fungal infections

  B. profound deficiencies of cell-mediated immunity

  C. depletion of lymphocytes in the paracortical areas of lymph nodes

  D. normal numbers of B lymphocytes

  E. very low quantities of immunoglobulin in their serum

  Correct Answer: E

  Section: Microbiology/Immunology Bruton hypogammaglobulinemia is a B-cell immunodeficiency disorder that is X-linked recessive and thus only affects males (boys). Affected patients are deficient in B cells in the peripheral blood and in B-dependent areas of lymph nodes and spleen. Most of the serum immunoglobulins are absent, and the IgG level is <200 mg/L. Recurrent pyogenic infections usually begin to occur at 56 months of age, when maternal IgG has been depleted. Individuals with Bruton syndrome have normal T-cell-mediated immune responses (choice B). Patients suffering from X-linked hypogammaglobulinemia do not have normal numbers of B lymphocytes, because the pre-B cells from which B cells are produced fail to differentiate into B cells. This is due to a gene mutation in pre-B cells, which does not allow pre-B cells to form tyrosine kinase (choice D). The number of lymphocytes in the paracortical areas of lymph nodes of patients with Bruton syndrome is normal (choice C). Individuals with X-linked hypogammaglobulinemia have normally functioning T cells, and thus are not particularly susceptible to viral or fungal infections, which are dependent on proper T-cellmediated immune responses (choice A).

• Question 430:

  A 7-year-old boy experienced respiratory tract inflammation, sore throat, and fever. Labored breathing soon followed the development of a gray membrane in the tonsil area, and diphtheria was diagnosed. Which of the following represents the most immediate course of action by his physician?

  A. acid-fast stain of a throat specimen

  B. culture of a throat specimen on blood agar

  C. injection of diphtheria antitoxin

  D. oral administration of sulfonamides

  E. performance of a spinal tap

  Correct Answer: C

  Section: Microbiology/Immunology A physician is justified in giving antitoxin on clinical evidence, or suspicion of diphtheria, without waiting for laboratory confirmation. The antitoxin dosage should be adjusted according to the weight of the patient and the severity of the infection. The antitoxin is given to neutralize free diphtheria exotoxin in the body fluids and timeliness is of extreme importance. Once the exotoxin has been bound by the body cells and exerted its influence, diphtheria antitoxin is of little value. C. diphtheria localizes in the throat, and thus spinal taps are useless (choice E). Tellurite agar, not blood agar, is used for the isolation of C. diphtheriae from throat swabs, because it is a selective medium for this germ, inhibiting the growth of other bacteria present in throat swabs (choice B). C. diphtheriae is not an acid-fast microbe. Methylene blue is used to stain smears for the bacteriological diagnosis of diphtheria (choice A). This initial treatment of choice for diphtheria is antitoxin. Treatment with penicillin G or erythromycin, but not sulfonamides, may be used.

  Penicillin G or erythromycin are not substitutes for diphtheria antitoxin (choice D).