**Core Concept**
The underlying principle being tested is the treatment of antibiotic-associated pseudomembranous colitis caused by **Clostridium difficile**. This condition occurs when the normal gut flora is disrupted, allowing **C. difficile** to overgrow and produce toxins that cause colitis.

**Why the Correct Answer is Right**
The correct answer is likely a drug that is effective against **C. difficile**, such as metronidazole or vancomycin. These drugs work by inhibiting the growth of **C. difficile** and reducing the production of toxins.

**Why Each Wrong Option is Incorrect**
**Option A:** May not be effective against **C. difficile**.
**Option B:** May not be effective against **C. difficile**.
**Option C:** May not be effective against **C. difficile**.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that **vancomycin** is often used to treat severe cases of **C. difficile** colitis, while **metronidazole** is used for mild to moderate cases.

**Correct Answer:** D. Vancomycin.