The correct answer is A. Enterococci are a common cause of nosocomial urinary tract infection and subacute endocarditis. Ampicillin is the standard treatment for susceptible enterococcal infections. For those strains that are not susceptible, vancomycin is the treatment of choice. For vancomycin-resistant strains, other therapies (such as quinupristin/dalfopristin) are available, but extensive knowledge of the indications for each is beyond the scope of knowledge necessary for the USMLE Step 1 examination.
Answer B is incorrect. Gentamicin is an aminoglycoside antibiotic. It inhibits the 30S subunit by inhibiting the formation of the initiation complex. It is not an effective monotherapy for enterococci, but it can be used in combination with other therapies to increase efficacy.
Answer C is incorrect. Piperacillin is an extended-spectrum penicillin agent. In addition to gram-positive organisms, it is particularly active against Pseudomonas aeruginosa and the Enterobacteriaceae viruses. It is not first-line therapy for enterococcal infections.
Answer D is incorrect. Quinupristin/dalfopristin (Synercid) is a new therapy that may be effective for vancomycin-resistant enterococcal endocarditis. It is not first-line treatment for enterococcal infections.
Answer E is incorrect. Vancomycin is an anti- biotic that is effective only against gram-posItive cocci. It binds tightly to a cell wall precursor containing the amino acid sequence D-ala D-ala, preventing cell wall synthesis. It is indicated for ampicillin-resistant enterococcal infection.