For a patient suspected of having infective endocarditis with a prosthetic aortic valve, what is the best empiric antimicrobial regimen?

The best empiric antimicrobial regimen for a patient suspected of having infective endocarditis with a prosthetic aortic valve includes ampicillin combined with nafcillin and gentamicin. This combination is effective against both Gram-positive organisms, including Staphylococcus aureus and Enterococcus species, which are common pathogens associated with infective endocarditis, especially in patients with prosthetic valves.

Ampicillin provides good coverage against Enterococcus, while nafcillin is effective against methicillin-sensitive Staphylococcus aureus (MSSA). Gentamicin is included to provide synergistic effect against Enterococci when combined with a beta-lactam, enhancing the killing effect of the antibiotics.

This regimen is particularly important in patients with prosthetic valves because they are at higher risk for infection by organisms that these agents can target, and the combination maximizes the likelihood of effective treatment in a potentially serious condition like infective endocarditis.

Other options, although they include useful agents, do not provide the same breadth of coverage or appropriate synergy necessary for treating prosthetic valve endocarditis effectively.