Drug of choice for MRSA infection –
**Core Concept**
The treatment of Methicillin-resistant Staphylococcus aureus (MRSA) infections involves the use of antibiotics that are effective against this pathogen. MRSA is resistant to beta-lactam antibiotics, including methicillin, due to the presence of the mecA gene, which encodes for an altered penicillin-binding protein (PBP2a) with low affinity for beta-lactam antibiotics.
**Why the Correct Answer is Right**
Vancomycin is a glycopeptide antibiotic that inhibits cell wall synthesis by binding to the D-alanyl-D-alanine terminus of cell wall precursors, preventing their incorporation into the peptidoglycan layer. This action is effective against MRSA, which lacks the beta-lactam resistance mechanism. Vancomycin is considered the drug of choice for severe MRSA infections, including bacteremia and endocarditis.
**Why Each Wrong Option is Incorrect**
**Option A:** Linezolid is an oxazolidinone antibiotic that inhibits protein synthesis by binding to the 50S ribosomal subunit. While effective against MRSA, linezolid is generally reserved for complicated skin and soft tissue infections or for patients with vancomycin intolerance.
**Option B:** Clindamycin is a lincosamide antibiotic that inhibits protein synthesis by binding to the 50S ribosomal subunit. However, MRSA resistance to clindamycin is often mediated by the ermC gene, which confers resistance through methylation of the ribosomal target.
**Option C:** Trimethoprim-sulfamethoxazole (TMP-SMX) is a combination antibiotic that inhibits folate synthesis by targeting dihydrofolate reductase and dihydropteroate synthase. While TMP-SMX is effective against some Gram-positive bacteria, it is not the preferred treatment for MRSA infections.
**Clinical Pearl / High-Yield Fact**
Vancomycin requires therapeutic drug monitoring to prevent nephrotoxicity, particularly at higher doses. A trough level of 15-20 mg/L is generally recommended to ensure efficacy and minimize toxicity.
**Correct Answer:** D. Vancomycin