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USMLE (National Board) Practice Examination, Block Two
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A 4-year-old girl has papular and pustular lesions on her face. The lesions are exuding a honey-colored serous fluid. You make a clinical diagnosis of impetigo. A Gram stain of the exudate reveals gram-positive cocci in chains, and a culture reveals β-hemolytic colonies on blood agar. For which one of the following sequelae is she MOST at risk?
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(C) Paralysis of the facial nerve (Bell’s palsy)
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(D) Red blood cells and albumin in her urine
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The purified genome of certain RNA viruses can enter a cell and elicit the production of progeny viruses (i.e., the genome is infectious). Regarding these viruses, which one of the following statements is MOST accurate?
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(A) They have a segmented genome.
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(B) They have a polymerase in the virion.
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(C) Their genome RNA is double-stranded.
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(D) They encode a protease that cleaves a precursor polypeptide.
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(E) Their genome RNA has the same base sequence as mRNA.
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A 77-year-old man with enterococcal endocarditis needed to be treated with penicillin G but had a history of a severe penicillin reaction. He was therefore skin tested using penicilloyl-polylysine as the antigen. Which one of the following is MOST likely to occur in a positive skin test?
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(A) The antigen forms immune complexes with IgG.
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(B) The antigen activates CD4-positive T cells and macrophages.
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(C) The antigen activates the alternative pathway of complement.
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(D) The antigen activates CD8-positive T cells by binding to class I MHC proteins.
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(E) The antigen cross-links IgE on the mast cells and causes the release of histamine.
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Regarding the Gram stain, which one of the following is the MOST accurate?
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(A) After adding crystal violet and Gram’s iodine, both gram-positive bacteria and gram-negative bacteria will appear blue.
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(B) If you forget to stain with the red dye (safranin or basic fuchsin), both gram-positive bacteria and gram-negative bacteria will appear blue.
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(C) If you forget to heat-fix, both gram-positive bacteria and gram-negative bacteria will appear blue.
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