Explanation: MSSA commonly makes a penicillinase and should be considered resistant to penicillin, amoxicillin, and ampicillin unless unique susceptibility testing confirms activity. Even when confirmed, penicillin is not recommended in some severe infections such as endocarditis due to reliability issues with confirmatory testing. Addition of a beta-lactamase in amoxicillin-clavulanate, ampicillin-sulbactam, and piperacillin-tazobactam restores activity against MSSA.