Which antimicrobial agent is recommended for screening MRSA?

• A. Erythromycin
• B. Oxacillin
• C. Cefoxitin
• D. Azithromycin

Answer: (C)

Screening for MRSA relies on detecting mecA-mediated resistance to beta-lactams. Cefoxitin is the best choice because it is a strong inducer of mecA expression and its inhibition pattern correlates most reliably with the presence of the PBP2a-related resistance mechanism. In practice, MRSA isolates show little to no inhibition around a cefoxitin disk, while susceptible, non-MRSA isolates display a clear zone; this makes cefoxitin disc testing a sensitive and specific surrogate for mecA-mediated resistance. Oxacillin can yield unreliable results due to heteroresistance and variable mecA expression, whereas macrolides like erythromycin or azithromycin do not address beta-lactam resistance in MRSA.