USMLE USMLE Certifications USMLE-STEP-1 Questions & Answers

• Question 431:

  Amedical student was bitten by infected ticks. In a week, he developed a high fever, headache, muscular aches, nausea, and splenomegaly. Five days later his symptoms subsided. However, 1 week later all his previous symptoms returned. During the next 9 days he went through a recovery and another relapse followed by a final recovery. The overall temperature curve of his illness is shown in below figure. Which of the following is the most likely etiological agent?

  

  A. Borrelia burgdorferi

  B. B. recurrentis

  C. Leptospira interrogans

  D. T. carateum

  E. T. pertenue

  Correct Answer: B

  Section: Microbiology/Immunology The endemic form of relapsing fever is transmitted by ticks infected by B. recurrentis. Relapsing fever begins with headache, high fever, muscle aches, and splenomegaly. It has a unique fever curve due to the emergence of various antigenic types of B. recurrentis. B. burgdorferi (choice A) causes a distinct spreading circular rash with a clear center, which is called erythema chronicum migrans. L. interrogans (choice C) is transmitted by rat urine and causes leptospirosis. This disease is associated with jaundice, uremia, and aseptic meningitis. T. carateum (choice D) is the cause of pinta, which is characterized by hyperpigmentation of the skin. T. pertenue (choice E) is transmitted by contact with infected persons and is characterized by the development of cauliflowerlike skin lesions.

• Question 432:

  Administration of which one of the following substances is likely to be most beneficial to this patient?

  A. gamma interferon

  B. interleukin-1

  C. plasmapheresis

  D. prednisone

  E. tumor necrosis factor (TNF)

  Correct Answer: D

  Section: Microbiology/Immunology Prednisone alone or in combination with methotrexate and azathioprime is commonly used for the treatment of systemic lupus erythematosus. The cytokines TNF (choice E), interleukin-1 (choice B), or gamma interferon (choice A) have not been shown to be effective against SLE. Plasmapheresis (choice C) has been used for the treatment of Guillain-Barre syndrome and myasthenia gravis

• Question 433:

  A 25-year-old female presents to her doctor's office with a rash over the malar eminences of her face, sparing the nasolabial folds; a skin rash due to exposure to sunlight; and painless nasopharyngeal ulcers. Suspecting an autoimmune disorder, her physician obtains an antinuclear antibody test of her blood and finds high titers of anti-double-stranded DNA antibodies. Which one of the following diseases is most likely responsible for this immunological reaction?

  A. Goodpasture syndrome

  B. multiple sclerosis

  C. myasthenia gravis

  D. rheumatoid arthritis

  E. systemic lupus erythematosus

  Correct Answer: E

  Section: Microbiology/Immunology SLE is characterized by development of auto-antibodies against double-stranded DNA, and other components of the nucleus. These auto-antibodies are involved in the production of immune complexes that lead to activation of complement, tissue damage, including vasculitis, glomerulonephritis, or arthritis. Rheumatoid arthritis (choice D) is due to production of auto-antibodies (of the IgM type) against IgG. In multiple sclerosis (choice B), auto-activated T cells and macrophages cause demyelization of the white matter of the brain. Goodpasture syndrome (choice A) is caused by the development of auto-antibodies to the collagen component of the basement membrane of the lungs and kidneys. In myasthenia gravis (choice C), auto-antibodies are generated against the acetylcholine receptors of the neuromuscular junction.

• Question 434:

  A newborn boy appears to be lethargic and septic. Aspinal tap was performed, and a Gram stain of the spinal fluid revealed gram-positive bacilli. Cultures of the spinal fluid on sheep blood agar plates, at a temperature of 2225°C, yielded catalase-positive he molytic rods that had a tumbling motion. Penicillin cleared the infection. Which of the following is the most likely organism that caused this disease?

  A. Bacillus cereus

  B. Bordetella pertussis

  C. Corynebacterium diphtheria

  D. Listeria monocytogenes

  E. Neisseria meningitidis

  Correct Answer: D

  Section: Microbiology/Immunology

  L. monocytogenes causes meningitis and sepsis in newborns and patients whose immune systems have been compromised by irradiation or chemotherapy. When this microbe is growing at a temperature of 2225°C on blood agar, it yields gram-positive rods, w hich move with a characteristic tumbling motion. A slight zone of hemolysis surrounds the colonies of L. monocytogenes. These characteristics are not applicable to B. cereus (choice A), B. pertussis (choice B), N. meningitides (choice E), or C.

  diphtheriae (choice C).

• Question 435:

  Below figure shows a quantitative precipitin curve of an antigen-antibody reaction, where the amount of antibody is kept constant throughout. According to the figure, which of the following statements is correct?

  

  A. point A shows antigen excess

  B. point B shows that there is no antibody formation

  C. point C indicates that there is little, if any, free antigen and antibody in the reaction tube

  D. point D represents the area of antigen destruction

  E. point E indicates antibody excess

  Correct Answer: C

  Section: Microbiology/Immunology Point C represents the zone of equivalence, that is, that point of the antigen-antibody reaction when optimal concentrations of antigen and antibody combine. Thus, there is little, if any, free antigen and antibody, and as a consequence maximal amounts of antigen and antibody will be found in the precipitate resulting from the antigen-antibody reaction. Points A and B represent areas in the antigen- antibody precipitin curve where there is an excess of antibody (choices A and B). Points D and E show areas in the precipitin curve where there is an antigen excess in the antigen-antibody reaction mixture (choices D and E).

• Question 436:

  The tetracycline antimicrobials are effective specifically in prokaryotes due to which of the following activities?

  A. They are transported into prokaryotes but not into eukaryotes.

  B. They bind to prokaryotic and not eukaryotic DNA-directed RNA polymerase.

  C. They bind to prokaryotic but not eukaryotic membranes.

  D. They bind to prokaryotic but not eukaryotic aminoacyl-tRNAs.

  E. They inhibit prokaryotic protein synthesis initiation, which specifically requires formylmethionyl tRNA.

  Correct Answer: A

  Section: Microbiology/Immunology The specificity of the tetracycline antibiotics is attributable to an energy-dependent transport system present in prokaryotes, but not in eukaryotic cells. This transport system results in accumulation of the drug inside the bacterial cell, where it binds to the ribosome and interferes with binding of aminoacyl tRNA to the acceptor site. Tetracyclines bind to the bacterial ribosome, but not to DNA-directed RNA polymerase, or prokaryotic membranes (choices B and C). Tetracyclines do not inhibit initiation of protein synthesis, which specifically requires formylmethionyl tRNA (choice E). Both prokaryotic and eukaryotic cells bind tetracyclines. However, the prokaryotic cells have much more effective tetracycline transport systems than eukaryotic cells. Thus, prokaryotic cells accumulate higher intracellular concentrations of tetracyclines than eukaryotic cells, which induce a significant inhibition of protein synthesis only in prokaryote cells (choice D).

• Question 437:

  What would be the appearance of invasives C. albicans in a Gram-stained abnormal vaginal smear?

  A. arthrospores-alternating filled and empty

  B. branching mycelia

  C. budding yeast cells and hyphae

  D. endosporulating spherules

  E. single yeast cells

  Correct Answer: C

  Section: Microbiology/Immunology Candida species are considered to be opportunistic microorganisms. They are normal or usual members of the skin, mucus membranes, and the GI tract. The risk of endogenous opportunistic infection is ever-present. In culture or tissue, candida species grow as oval, budding yeast cells (36 m) which stain blue in the Gram-stain procedure. They also form pseudohyphae when the buds grow, but fail to detach. C. albicans is dimorphic, being also able to produce true hyphae. Diagnostically, C. albicans will begin to form true hyphae (germ tubes) in serum at 37°C in the la boratory. With these characteristics in mind, choice C is correct. Alternating filled and empty arthrospores (choice A) is characteristic of coccidioides. Branching mycelia (choice B) is a general characteristic seen in any mold colony on agar and is not specific for candida. Endosporulating spherules (choice D) are also seen in tissue with coccidioides. Single yeast cells (choice E) may be occasionally seen with candida, but most candida spores will be associated with buds and pseudohyphae.

• Question 438:

  Viruses have various ways of entering the human body and producing disease. Which of the following descriptions accurately describe the route and mechanism for the virus indicated?

  A. Coronaviruses enter the gastrointestinal tract through the mouth and move into the stomach, where they proliferate within mucosal cells to produce peptic ulcers.

  B. Enteroviruses enter through the mouth, replicate in the pharynx and bowel, and move via the blood to distant target organs (central nervous system).

  C. HIV is directly injected into the blood stream by insects requiring blood for egg development.

  D. Influenza viruses enter through the respiratory tract, replicate within lymphocytes in the lung, and move via the lymphatic vessels to joints and the central nervous system to produce muscle aches, stiff joints, and fever.

  E. Mumps virus enters through abraded skin in the genital area and moves into the testicles of males to produce swelling and sterility.

  Correct Answer: B

  Section: Microbiology/Immunology It is important that physicians have a good understanding of how pathogens, including viruses, enter into the human host and know what symptoms may be caused by the microorganism. Such knowledge and interpretations may have significant value in making a meaningful differential diagnosis. Enteroviruses do enter through the mouth, replicate in the pharynx and bowel before using the blood system (viremia) to access final target organs (choice B). Coronaviruses (choice A) are inhaled or ingested (small number of strains), but do not produce peptic ulcers. Insects have been shown to not be important for HIV transmission (choice C). Influenza viruses (choice D) do not replicate in lung lymphocytes. Mumps virus (choice E) is most often inhaled from respiratory secretions from infected individuals who make large numbers of the viruses in the upper respiratory tract during the prodromal period.

• Question 439:

  On November 6, a patient had the onset of an illness characterized by fever, chills, headache, cough, and chest pain. The illness lasted 1 week. On December 5, she had another illness very similar to the first, which lasted 6 days. She had no flu immunization during this period. Her Hl titer to swine flu virus is shown in below figure. (There were no laboratory errors in any of the tests.) Which of the following is the most valid conclusion?

  

  A. It is impossible to relate either illness with the specific virus.

  B. The patient was ill with swine flu later, and the November 6 illness was due to another pathogen.

  C. The patient was ill with swine flu on December 20.

  D. The patient was ill with swine flu on November 6.

  Correct Answer: B

  Section: Microbiology/Immunology Influenza viruses are classified into three main groups (A, B, and C) based on proteins in the ribonucleoprotein capsid. Each virus can be further subdivided into various groups when the hemeagglutinin (HA) and NA glycoproteins are examined. So far, 15 subtypes of HA (H1H15) and 9 subtypes of NA have been identified, and these can be found in many different combinations for influenza virus types or strains. Influenza viruses also have the ability to attach to and to agglutinate various types of red blood cells, including chicken RBCs. Antibodies made against HA antigens will neutralize HA activity and inhibit HA. This HI data can be used clinically to determine whether the same or different flu viruses have caused single or multiple outbreaks. In our clinical episode in this question and below figure, an HI titer of 10 (or 1:10 dilution of the patient's serum) represents little or no specific antibody against the specific virus, (swine flue virus) used in the HI test. Infection may have occurred in the past, with only small amounts of specific antibody remaining. The 160 (1:160) titer indicates a large amount of specific anti-swine flu immunoglobulins are now present. Apositive diagnosis for swine flu infection by December 20 is stated because it is fourfold or higher than the November 30 serum sample. As a result, choice B is the proper interpretation of these serological test results. Choice A is incorrect because we have specifically measured antibodies against swine flu on December 20. Choice C is partially correct but does not interpret results for November 6 and 30. Choice D is specifically incorrect since only swine flu was used in this HI test and the November 6 data is essentially negative.

• Question 440:

  Which of the following chemical agents is a powerful oxidizing agent used to kill microorganisms?

  A. benzoic acid

  B. chlorine

  C. ethyl alcohol

  D. ethylene oxide

  E. mercury

  Correct Answer: B

  Section: Microbiology/Immunology Chlorine (choice B) is a halogen that is often used for microbial disinfection and antisepsis. Many compounds can release various forms of chlorine (OCl-, HOCl, Cl2) that can kill microorganisms by oxidation. These include sodium hypochlorite, chlorine dioxide, and sodium dichlaroisocyanrate, oxidizing agents that destroy the cellular activity of proteins. Hypochlorous acid is the active ingredient for bactericidal and viruscidal effects. Such compounds may be sporocidal. Iodine (I2) may also be bactericidal, fungicidal, tuberculocidal, viruscidal, and sporocidal. Mercury (choice E) is antimicrobial but dangerous to humans. Ethanol (choice C) has limited killing ability for bacteria. Benzoic acid (choice A) has limited antimicrobial use, which ethylene oxide (choice D) is able to sterilize (completely kill all organisms) materials, but is very dangerous to humans as well.