**Core Concept**
Pseudomembranous colitis, also known as Clostridioides difficile (C. difficile) colitis, is a type of inflammatory bowel disease caused by an overgrowth of bacteria in the colon. It is often associated with antibiotic use, which disrupts the normal balance of gut flora.

**Why the Correct Answer is Right**
The most commonly implicated antibiotic is a broad-spectrum antibiotic that can disrupt the normal balance of gut flora, allowing C. difficile to overgrow. C. difficile produces toxins A and B, which cause inflammation and damage to the colonic mucosa, leading to pseudomembranous colitis. The correct answer is a broad-spectrum antibiotic that can lead to this disruption.

**Why Each Wrong Option is Incorrect**
**Option A:** Metronidazole is an antibiotic that is used to treat C. difficile colitis, not cause it. It is actually used as a treatment option for pseudomembranous colitis.

**Option B:** Vancomycin is another antibiotic that is used to treat C. difficile colitis, but it is not commonly associated with the development of pseudomembranous colitis.

**Option C:** Lactulose is not an antibiotic, but rather a medication used to treat constipation and hepatic encephalopathy. It is not associated with the development of pseudomembranous colitis.

**Clinical Pearl / High-Yield Fact**
A mnemonic to remember the most common antibiotics associated with pseudomembranous colitis is "CLOSTRIDIUM": Ciprofloxacin, Lincomycin, Ofloxacin, Streptogramins, Tigecycline, Imipenem, Rifampin, and Daptomycin.

**Correct Answer:** C. Ciprofloxacin.