**Core Concept**
Pseudomembranous colitis, also known as Clostridioides difficile (C. diff) infection, is a type of colitis caused by an overgrowth of the bacterium C. diff in the colon. Antibiotic-associated pseudomembranous colitis is a complication of antibiotic use that disrupts the normal gut flora, allowing C. diff to overgrow and cause infection.

**Why the Correct Answer is Right**
Vancomycin is the drug of choice for treating antibiotic-associated pseudomembranous colitis because it is a bactericidal antibiotic that is effective against C. diff. Vancomycin works by inhibiting cell wall synthesis in C. diff, ultimately leading to the death of the bacteria. Vancomycin is not absorbed from the gastrointestinal tract, allowing it to remain effective in the colon where C. diff is present.

**Why Each Wrong Option is Incorrect**
**Option A:** Metronidazole is an alternative treatment for C. diff infection, but it is not the first-line treatment. It is less effective than vancomycin and has a higher risk of treatment failure.
**Option B:** Ciprofloxacin is a fluoroquinolone antibiotic that can actually exacerbate C. diff infection by further disrupting the normal gut flora.
**Option C:** Amoxicillin-clavulanate is a broad-spectrum antibiotic that can also disrupt the normal gut flora and make C. diff infection worse.

**Clinical Pearl / High-Yield Fact**
Vancomycin should be taken orally for the full treatment course, even if symptoms improve before completing treatment, to ensure complete eradication of C. diff.

**Correct Answer:** C. Vancomycin