A 40-year-old IV drug user develops fever and wrist pain. Blood cultures grow Staphylococcus aureus. Joint aspiration shows WBC 120,000/μL. After initiating antibiotics and joint drainage, which antibiotic is recommended as first-line for methicillin-sensitive S. aureus (MSSA) septic arthritis?

• A Vancomycin
• B Flucloxacillin (or oxacillin/nafcillin) ✓
• C Meropenem
• D Linezolid

Explanation

For MSSA infections, antistaphylococcal penicillins (flucloxacillin, oxacillin, or nafcillin) are first-line — they achieve superior bactericidal activity compared to vancomycin against MSSA. Vancomycin is reserved for MRSA infections or penicillin-allergic patients. Meropenem is a broad-spectrum carbapenem not routinely needed for MSSA. Linezolid has bacteriostatic activity against S. aureus and is not first-line for joint infections. The choice between de-escalating from empirical vancomycin to flucloxacillin once sensitivity is confirmed is a standard stewardship decision.

Reference: Maheshwari Essential Orthopaedics, 6th ed.

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Written and medically reviewed by the StethoPrep medical team.