**Core Concept**
Clostridium difficile-associated diarrhea (CDAD) is a type of antibiotic-associated diarrhea caused by the overgrowth of C. difficile bacteria in the gut. Treatment involves restoring the balance of gut microbiota and eliminating the causative agent.

**Why the Correct Answer is Right**
Vancomycin is a lipoglycopeptide antibiotic that is effective against C. difficile due to its ability to bind to the bacterial cell wall, inhibiting cell wall synthesis. In the context of CDAD, vancomycin is administered orally to target the C. difficile bacteria directly in the gut, without being absorbed systemically. This approach helps to restore the balance of gut microbiota and alleviate symptoms.

**Why Each Wrong Option is Incorrect**
* **Option A:** Metronidazole is an antibiotic that targets anaerobic bacteria but is not the preferred treatment for CDAD, especially in cases of severe or recurrent disease.
* **Option B:** Piperacillin-tazobactam is a broad-spectrum cephalosporin antibiotic, but it is not effective against C. difficile.
* **Option D:** Ciprofloxacin is a fluoroquinolone antibiotic that can actually contribute to the development of CDAD due to its broad-spectrum activity and disruption of the gut microbiota.

**Clinical Pearl / High-Yield Fact**
When treating CDAD, it is essential to discontinue the use of broad-spectrum antibiotics that may have contributed to the development of the infection and to initiate treatment with vancomycin or fidaxomicin.

**Correct Answer:** C. Vancomycin.