**Core Concept**
The question requires knowledge of the treatment options for Clostridioides difficile (Cl. difficile) infection, a type of bacterial diarrhea caused by antibiotic use. The correct answer involves understanding the pharmacological principles of antimicrobial therapy and the pathophysiology of Cl. difficile infection.

**Why the Correct Answer is Right**
The drug of choice for treating Cl. difficile infection is fidaxomicin. Fidaxomicin is a macrocyclic antibiotic that targets the bacterial ribosome, inhibiting protein synthesis in Cl. difficile. It has a narrow spectrum of activity, which helps reduce the risk of developing antibiotic resistance. Fidaxomicin also has a high cure rate and a favorable safety profile, making it a preferred treatment option.

**Why Each Wrong Option is Incorrect**
* **Option A:** Metronidazole is an alternative treatment for mild to moderate Cl. difficile infection, but it is not the preferred choice due to concerns about antibiotic resistance and lower cure rates compared to fidaxomicin.
* **Option B:** Vancomycin is an option for severe Cl. difficile infection, but it is not the preferred choice for initial treatment due to its potential for causing colitis and the need for oral administration.
* **Option C:** Ciprofloxacin is a broad-spectrum antibiotic that can exacerbate Cl. difficile infection by disrupting the gut microbiota.

**Clinical Pearl / High-Yield Fact**
Fidaxomicin is specifically indicated for the treatment of Cl. difficile infection, and its use should be considered in patients with severe or recurrent infection.

**Correct Answer: B. Vancomycin is an option for severe Cl. difficile infection, but it is not the preferred choice for initial treatment due to its potential for causing colitis and the need for oral administration.