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
Drug of choice in methicillin-resistant Staphylococcus aureus - 
Select and explain. 
A. Ceftriaxone, B. Ceftazidime, C. Ampicillin, D. Vancomycin
D: Vancomycin
Answer: D: Vancomycin 
Explanation: 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)