**Core Concept**
The treatment of Clostridium difficile associated diarrhea involves the use of antibiotics that target the causative organism without exacerbating the condition. Vancomycin, a glycopeptide antibiotic, is effective against Clostridium difficile by inhibiting cell wall synthesis.

**Why the Correct Answer is Right**
Vancomycin acts by binding to the D-alanyl-D-alanine terminus of cell wall precursors, thereby inhibiting the enzyme transpeptidase and preventing cell wall synthesis. This mechanism of action is specific to gram-positive bacteria, making vancomycin effective against Clostridium difficile. In addition, vancomycin is not absorbed from the gastrointestinal tract, allowing it to act locally and reduce the risk of systemic side effects.

**Why Each Wrong Option is Incorrect**
**Option A:** Metronidazole is an antibiotic that targets anaerobic bacteria, but it is not the first-line treatment for Clostridium difficile associated diarrhea due to its potential for side effects and lower efficacy compared to vancomycin.
**Option B:** Ciprofloxacin is a fluoroquinolone antibiotic that targets a broad range of bacteria, but it is not effective against Clostridium difficile and may even exacerbate the condition.
**Option C:** Ampicillin is a beta-lactam antibiotic that targets gram-positive and gram-negative bacteria, but it is not effective against Clostridium difficile and may even promote the growth of resistant organisms.

**Clinical Pearl / High-Yield Fact**
Vancomycin is the preferred treatment for Clostridium difficile associated diarrhea due to its high efficacy and low risk of systemic side effects. It is essential to note that vancomycin is not absorbed from the gastrointestinal tract, making it a safe choice for patients with impaired renal function.

**Correct Answer:** C. Vancomycin.