**Core Concept**
Pseudomembranous colitis, also known as Clostridioides difficile (C. difficile) colitis, is a type of bacterial infection that affects the colon. It is characterized by the formation of a pseudomembrane in the colon, which is a thick, yellowish-green layer of inflammatory tissue.

**Why the Correct Answer is Right**
C. difficile is a gram-positive, spore-forming bacterium that is commonly found in the environment and can be transmitted through contaminated food, water, or surfaces. When the normal gut flora is disrupted, often due to antibiotic use, C. difficile can overgrow and produce toxins that cause inflammation and damage to the colon lining. This leads to the formation of a pseudomembrane and symptoms such as diarrhea, abdominal pain, and fever.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it is not a known causative agent of pseudomembranous colitis. While other bacteria may cause colitis, C. difficile is the primary cause of pseudomembranous colitis.
**Option B:** This option is incorrect because while it is a common cause of bacterial overgrowth, it is not the primary cause of pseudomembranous colitis. C. difficile is a specific bacterium that causes this condition.
**Option C:** This option is incorrect because it is not a known causative agent of pseudomembranous colitis. While other viruses may cause gastrointestinal symptoms, C. difficile is the primary cause of pseudomembranous colitis.

**Clinical Pearl / High-Yield Fact**
C. difficile is a major cause of healthcare-associated infections, and its diagnosis is often suspected in patients with diarrhea and a recent history of antibiotic use. It is essential to use appropriate diagnostic tests and infection control measures to prevent the spread of C. difficile.

**Correct Answer: D. Clostridioides difficile.**