A  

MRSA possesses the mecA-encoded PBP2a, which confers class-wide resistance to nearly all β-lactams, including second-generation cephalosporins such as cefaclor; therefore cefaclor is ineffective. In contrast, vancomycin remains the standard intravenous therapy, and many community strains retain susceptibility to cotrimoxazole, while fluoroquinolone activity is variable but not intrinsically absent [Watanakunakorn, 1982, PMID 7114635; Bal & Gould, 2005, PMID 16218886].