## **Core Concept**
Vancomycin is an antibiotic used to treat various bacterial infections. In the context of pseudomembranous colitis, which is often caused by *Clostridioides difficile* (C. diff), the goal is to deliver the antibiotic directly to the site of infection in the colon. Pseudomembranous colitis, also known as antibiotic-associated colitis, occurs when the normal gut flora is disrupted, allowing C. diff to overgrow and cause disease.

## **Why the Correct Answer is Right**
The correct route of administration for vancomycin in treating pseudomembranous colitis is oral. When given orally, vancomycin is not absorbed well from the gastrointestinal tract, which is advantageous because it allows high concentrations of the antibiotic to reach the colon, where C. diff resides. This localized high concentration is crucial for effectively reducing the bacterial load without significant systemic absorption, which could lead to toxicity.

## **Why Each Wrong Option is Incorrect**
- **Option A:** Intravenous administration is incorrect because vancomycin given intravenously is primarily excreted by the kidneys and achieves very low concentrations in the gut lumen, making it ineffective for treating C. diff infections in the colon.
- **Option B:** Intramuscular administration is also incorrect for similar reasons as intravenous; it does not achieve therapeutic concentrations in the colon and is not used for treating gastrointestinal infections like pseudomembranous colitis.
- **Option D:** Topical administration might seem plausible for localized infections, but vancomycin is not typically applied topically for pseudomembranous colitis. The drug needs to be in the lumen of the colon to be effective.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that oral vancomycin is a first-line treatment for *C. difficile* infection (CDI), which causes pseudomembranous colitis. It is specifically indicated for the treatment of CDI but not for other types of bacterial infections that might be treated with vancomycin systemically.

## **Correct Answer:** . Oral