**Core Concept:** MRSA (methicillin-resistant Staphylococcus aureus) and VRSA (vancomycin-resistant S. aureus) are strains of Staphylococcus aureus (S. aureus) that have developed resistance to certain antibiotics. Vancomycin is a last-resort antibiotic used to treat infections caused by MRSA and VRSA due to its ability to cross the bacterial cell wall and bind to D-Ala-D-ala segments of the peptidoglycan.

**Why the Correct Answer is Right:** Vancomycin is a glycopeptide antibiotic that works by inhibiting the synthesis of bacterial cell wall peptidoglycan. In the case of MRSA and VRSA, their resistance to beta-lactam antibiotics (methicillin, oxacillin, etc.) has led to their reliance on glycopeptides like vancomycin. Vancomycin can bind to D-Ala-D-ala segments of the peptidoglycan, disrupting the cell wall synthesis and causing cell lysis.

**Why Each Wrong Option is Incorrect:**

A. Vancomycin is not specifically designed to target MRSA or VRSA. It is a broad-spectrum antibiotic that can inhibit the growth of gram-positive bacteria, including S. aureus. However, its effectiveness against MRSA and VRSA depends on the bacteria's ability to synthesize peptidoglycan.

B. Linezolid is a newer oxazolidinone antibiotic that inhibits bacterial protein synthesis by binding to the 23S rRNA of the 50S ribosomal subunit. While it is effective against MRSA and vancomycin-resistant enterococci (VRE), it is not as effective against VRSA.

C. Daptomycin is a lipopeptide antibiotic that acts against gram-positive bacteria by binding to the cytoplasmic membrane and causing the loss of membrane potential and leakage of ions. Daptomycin is effective against MRSA and VRE but not specifically against VRSA due to its reliance on vancomycin for treatment.

D. Tigecycline is a glycylcycline antibiotic that inhibits bacterial protein synthesis by binding to the 50S ribosomal subunit. While it has activity against MRSA and VRE, its effectiveness against VRSA is limited.

**Clinical Pearl:** In cases where MRSA or VRE infection is suspected, consider using daptomycin or tigecycline as alternatives to vancomycin. However, in the context of VRSA, vancomycin remains the primary choice due to its ability to target the D-Ala-D-ala segments in the cell wall synthesis of gram-positive bacteria.