**Core Concept**
The patient presents with symptoms of pseudomembranous colitis, characterized by watery diarrhea, ulcers, plaques, and pseudomembranes in the colon. The diagnosis was made on stool cytotoxin assay, indicating a toxin-mediated infection. The patient's improvement with metronidazole suggests an anaerobic bacterial infection.

**Why the Correct Answer is Right**
Pseudomembranous colitis is often caused by Clostridioides difficile (C. difficile) toxin, which disrupts the normal gut microbiota and leads to inflammation. The appearance of ulcers, plaques, and pseudomembranes is a hallmark of C. difficile infection. The use of metronidazole, an antibiotic effective against anaerobic bacteria, supports this diagnosis. The CECT findings likely reveal thickening of the colonic wall, which is a characteristic feature of pseudomembranous colitis.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not describe a characteristic sign of pseudomembranous colitis.
**Option B:** This option is incorrect because it does not match the CECT findings described in the scenario.
**Option C:** This option is incorrect because it does not accurately describe the CECT findings associated with pseudomembranous colitis.

**Clinical Pearl / High-Yield Fact**
The "thumbprinting" sign on CECT, seen in pseudomembranous colitis, is due to submucosal edema and hemorrhage, which causes the colon wall to appear thickened and "thumb-like" on imaging.

**Correct Answer:** C. Thumbprinting sign