This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Infection, Subject name: Medicine
[{'content': 'Drug of choice in methicillin-resistant Staphylococcus aureus - \nSelect and explain. \nA. Ceftriaxone, B. Ceftazidime, C. Ampicillin, D. Vancomycin', 'role': 'user'}, {'content': "Answer: D \nExplanation: Methicillin resistance in S. aureus is determined by the mecA gene, which encodes for resistance to all b-lactam drugs. If CA-MRSA infection is suspected, either linezolid or vancomycin should be added to the initial empirical regimen. (Harrison's Principles of internal medicine, 20th edition)", 'role': 'assistant'}]