A 35-year-old IV drug user presents with fever (39.5°C), new-onset tricuspid regurgitation murmur, and bilateral pulmonary nodular opacities on CXR. Blood cultures grow Staphylococcus aureus (MSSA). TEE confirms a large mobile vegetation on the tricuspid valve. He has no prosthetic valve and no prior endocarditis. What is the most appropriate antibiotic regimen?

• A Vancomycin for 6 weeks
• B Daptomycin 6 mg/kg IV for 4–6 weeks
• C Cefazolin alone for 4 weeks
• D Nafcillin + gentamicin for 2 weeks ✓

Explanation

For right-sided (tricuspid) native valve MSSA endocarditis in IV drug users, the AHA guideline supports a 2-week regimen of nafcillin (or oxacillin) plus gentamicin — a validated short-course regimen with cure rates >90% for uncomplicated right-sided disease without hemodynamic compromise, CNS emboli, or metastatic infections. This shorter regimen is specific to right-sided MSSA endocarditis and does not apply to left-sided or MRSA infections. Vancomycin is less bactericidal than nafcillin for MSSA and is reserved for penicillin-allergic patients or MRSA.

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

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Written and medically reviewed by the StethoPrep medical team.