**Core Concept**
MRSA (Methicillin-resistant Staphylococcus aureus) is a type of bacterial infection resistant to many antibiotics, including methicillin and other penicillins. The drug of choice for MRSA infection should ideally be effective against this resistant strain.

**Why the Correct Answer is Right**
The preferred treatment for MRSA is vancomycin, a glycopeptide antibiotic. Vancomycin works by inhibiting cell wall synthesis in bacteria, ultimately leading to cell death. It has a broad spectrum of activity, including against MRSA, and is often the first-line treatment for severe infections.

**Why Each Wrong Option is Incorrect**
* **Option A:** Fluoroquinolones, such as ciprofloxacin, may not be effective against MRSA due to resistance mechanisms.
* **Option B:** Macrolides, like erythromycin, are not the preferred choice for MRSA infections as they may not provide adequate coverage.
* **Option C:** Trimethoprim-sulfamethoxazole (TMP-SMX) may be used in certain cases, but it is not the first-line treatment for MRSA due to potential resistance and side effects.

**Clinical Pearl / High-Yield Fact**
Vancomycin is often administered intravenously in a dose-adjusted manner to maintain therapeutic levels and minimize nephrotoxicity.

**Correct Answer:** D. Vancomycin.