Choice A. C difficile is largely noninvasive; pathology is mediated by the release of exotoxins (toxin A and toxin B) that cause inflammation and mucosal injury and lead to colonic ulceration. Although perforation can occur due to toxic megacolon, free air is usually seen on abdominal x-ray.
Choice B. Colonic pseudoobstruction, a disorder of bowel motility leading to acute colonic dilatation in the absence of a obstructing lesion, often occurs due to severe illness, abdominal surgery, or medication. Although abdominal distension and tenderness are common, fever, tachycardia, leukocytosis, and a history of diarrhea would all be atypical.
Choice C. Mechanical bowel obstruction usually occurs due to entanglement of bowel around fibrous strictures from previous abdominal operations. Patients often develop crampy abdominal pain and distension, and x-ray generally shows a tapered transition point (where the distal bowel is pinched by the obstruction). Leukocytosis, fever, tachycardia, and preceding diarrhea are atypical.
Educational objective: Clostridioides (formerly Clostridium difficile) infections can occasionally be complicated by toxic megacolon, which usually presents with severe systemic symptoms (eg, high fever, tachycardia), leukocytosis, abdominal distension, and significant colonic distension on abdominal radiograph. Suspicion is often raised when a patient with C difficile infection stops having diarrhea and symptoms clinically worsen.
Choice E. Ischemic colitis is caused by a drop in colonic perfusion pressure (eg, arterial embolism or thrombus). Most cases manifest with mild, cramping lower abdominal pain, bloody diarrhea, and/or hematochezia. Although colonic distension can be seen in advanced cases, abdominal radiographs are usually normal (thumbprinting [submucosal edema] is sometimes seen).
