**Core Concept**
Vancomycin is a glycopeptide antibiotic used to treat serious bacterial infections, particularly those caused by Gram-positive bacteria. It works by inhibiting cell wall synthesis, ultimately leading to bacterial cell death.

**Why the Correct Answer is Right**
Vancomycin is used to treat a range of infections, including methicillin-resistant Staphylococcus aureus (MRSA), Clostridioides difficile (C. difficile), and endocarditis. It is often administered intravenously for severe infections or orally for C. difficile colitis. The optimal serum trough concentration of vancomycin is typically between 15-20 mg/L to prevent nephrotoxicity and optimize efficacy.

**Why Each Wrong Option is Incorrect**
**Option A:** Vancomycin is not used to treat infections caused by Gram-negative bacteria.
**Option B:** Vancomycin is not effective against methicillin-sensitive Staphylococcus aureus (MSSA) infections; it is specifically used for MRSA.
**Option C:** Vancomycin is not used as a first-line treatment for uncomplicated skin infections; it is reserved for more severe, complicated, or resistant infections.

**Clinical Pearl / High-Yield Fact**
Vancomycin requires monitoring of serum trough concentrations to prevent nephrotoxicity and optimize efficacy. A trough concentration of 15-20 mg/L is generally considered safe and effective.

**Correct Answer: B. Vancomycin is not effective against methicillin-sensitive Staphylococcus aureus (MSSA) infections; it is specifically used for MRSA.**