A 40-year-old IV drug user develops fever, new holosystolic murmur at the left sternal border, and multiple septic pulmonary emboli on CT. Blood cultures grow Staphylococcus aureus (MSSA). Echo shows a 15 mm mobile vegetation on the tricuspid valve. What is the treatment of choice for right-sided S. aureus endocarditis in this setting?
- A IV vancomycin for 6 weeks
- B IV daptomycin for 6 weeks
- C IV nafcillin (or flucloxacillin) for 4–6 weeks ✓
- D Oral rifampicin + trimethoprim-sulfamethoxazole for 4 weeks
Explanation
Right-sided native valve MSSA endocarditis (typically tricuspid valve in IV drug users) is treated with IV beta-lactam (nafcillin or flucloxacillin 12 g/day) for 4–6 weeks as the gold standard — beta-lactams are superior to vancomycin for MSSA and significantly reduce relapse rates. A shorter 2-week course of nafcillin + aminoglycoside was validated in small trials for uncomplicated right-sided MSSA IE without extrapulmonary emboli, renal failure, or MRSA. Daptomycin is an alternative if beta-lactam intolerant. Vancomycin is reserved for MRSA. Surgery is indicated for persistent bacteremia, vegetation >20 mm, or failure to respond.
Reference: Harrison's Principles of Internal Medicine, 21st ed.
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Written and medically reviewed by the StethoPrep medical team.