A 34-year-old intravenous drug user presents with fever, new pansystolic murmur loudest at the left sternal edge increasing with inspiration, and multiple pulmonary cavitating infiltrates. Blood cultures grow Staphylococcus aureus (MSSA). What is the most likely affected valve and the empirical antibiotic of choice?

• A Tricuspid valve; IV cloxacillin (or nafcillin) for 6 weeks ✓
• B Mitral valve; vancomycin for 4 weeks
• C Aortic valve; IV gentamicin plus penicillin
• D Pulmonary valve; IV daptomycin for 6 weeks

Explanation

Right-sided infective endocarditis in IV drug users most commonly involves the tricuspid valve. Septic pulmonary emboli (cavitating pulmonary infiltrates) are the hallmark of tricuspid IE. MSSA (oxacillin-sensitive) is best treated with anti-staphylococcal penicillin (cloxacillin/nafcillin) for 4–6 weeks, superior to vancomycin for MSSA (inferior pharmacodynamics). Vancomycin is reserved for MRSA. Daptomycin is a second-line alternative for right-sided MSSA/MRSA endocarditis. Pulmonary valve IE is rare even in IVDU.

Reference: Harrison's Principles of Internal Medicine, 21st ed.

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