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USMLE (National Board) Practice Examination, Block Two

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?

(A) Bloody diarrhea

(B) Blurred vision

(C) Paralysis of the facial nerve (Bell’s palsy)

(D) Red blood cells and albumin in her urine

(E) Rusty-colored sputum

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?

(A) They have a segmented genome.

(B) They have a polymerase in the virion.

(C) Their genome RNA is double-stranded.

(D) They encode a protease that cleaves a precursor polypeptide.

(E) Their genome RNA has the same base sequence as mRNA.

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?

(A) The antigen forms immune complexes with IgG.

(B) The antigen activates CD4-positive T cells and macrophages.

(C) The antigen activates the alternative pathway of complement.

(D) The antigen activates CD8-positive T cells by binding to class I MHC proteins.

(E) The antigen cross-links IgE on the mast cells and causes the release of histamine.

Regarding the Gram stain, which one of the following is the MOST accurate?

(A) After adding crystal violet and Gram’s iodine, both gram-positive bacteria and gram-negative bacteria will appear blue.

(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.

(C) If you forget to heat-fix, both gram-positive bacteria and gram-negative bacteria will appear blue.

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