**Core Concept**
MRSA (Methicillin-resistant Staphylococcus aureus) is a type of bacterial infection caused by a strain of Staphylococcus aureus that is resistant to many antibiotics, including methicillin and other penicillins. Treatment of MRSA often involves the use of alternative antibiotics that are effective against this resistant strain.

**Why the Correct Answer is Right**
Vancomycin is a glycopeptide antibiotic that is commonly used to treat MRSA infections. It works by inhibiting cell wall synthesis in bacteria, ultimately leading to cell lysis and death. Vancomycin is effective against MRSA due to its ability to bind to the bacterial cell wall, preventing the incorporation of new peptidoglycan layers.

**Why Each Wrong Option is Incorrect**
**Option A:** Methicillin is actually the antibiotic that MRSA is resistant to, making it an incorrect choice for treating MRSA.
**Option B:** No information was provided for this option, but in general, if the option is missing, we can't evaluate it. However, we can say that the other options are known treatments for MRSA, so the missing option might be a treatment for something else.
**Option C:** No information was provided for this option, but it's possible that this option is a treatment for something else or is not used for MRSA.

**Clinical Pearl / High-Yield Fact**
It's essential to note that the use of vancomycin for MRSA is particularly relevant in the context of hospital-acquired infections, where the risk of antibiotic resistance is higher.

**Correct Answer:** A. Methicillin