A 74-year-old man is brought to the emergency department due to worsening lethargy and abdominal pain and distension. For the past several days, the patient has had watery diarrhea. However, in the last 24 hours, he has not had a bowel movement, and the abdominal pain and distension have worsened, with the patient becoming progressively more lethargic. Medical history is significant for hypertension, myocardial infarction, atrial fibrillation, and stroke with hemiplegia. He was recently hospitalized for treatment of an infected pressure ulcer. Temperature is 38.8 C (101.8 F), blood pressure is 106/60 mm Hg, and pulse is 118/min. On physical examination, the patient is ill appearing and somnolent. Mucous membranes are dry. The abdomen is distended and diffusely tender. Bowel sounds are decreased. Leukocyte count is 18,000/mm³. Serum potassium is 3.2 mEq/L. Abdominal x-ray is shown in the image below: Which of the following is the most likely cause of this patient's current condition?

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Explanation: This patient's watery diarrhea, abdominal distension, leukocytosis, fever, and radiographic evidence of a markedly distended colon raise strong suspicion for toxic megacolon due to Clostridioides difficile infection. Major risk factors for C difficile include advanced age, recent hospitalization, and antibiotic use (eg, for infected pressure ulcers). Most cases of C difficile colitis are marked by watery diarrhea (≥3 in 24 hours), abdominal pain, low-grade fever, and nausea. A minority of patients develop severe complications such as toxic megacolon, which is characterized by nonobstructive colonic dilation. Toxic megacolon generally presents with the following: Severe systemic toxicity (eg, fever, hypotension, tachycardia, lethargy) Abdominal distension and pain (diarrhea may cease due to lack of colonic motility) Leukocytosis (usually >15,000/mm³) Radiographic evidence of large bowel dilation (eg, >6 cm [60 mm] in colon, >9 cm [90 mm] in cecum) Treatment includes bowel rest, nasogastric tube placement, and aggressive antibiotic therapy against C difficile. Any agent that contributes to a lack of gastrointestinal motility (eg, opiates, anticholinergics, antimotility medications) should be discontinued. Lack of response or clinical deterioration often necessitates further imaging (eg, CT scan of the abdomen) and, sometimes, surgical intervention (eg, colectomy). (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 x-ray. free air (Choice B) Hypokalemia can cause muscle weakness leading to ileus when it is severe (eg, <2.5-3.0 mEq/L). However, mild hypokalemia (eg, 3.0-3.4 mEq/L), as in this patient (likely from his initial diarrhea), does not usually produce skeletal or smooth muscle weakness. (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 often shows a tapered transition point (where the distal bowel is pinched by the obstruction). Leukocytosis, fever, tachycardia, and preceding diarrhea are atypical. (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). Educational objective: Clostridioides 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. References: Toxic megacolon associated Clostridium difficile colitis. (http://www.ncbi.nlm.nih.gov/pubmed/21160629) ACG clinical guidelines: prevention, diagnosis, and treatment of Clostridioides difficile infections. (http://www.ncbi.nlm.nih.gov/pubmed/34003176)

A 74-Year-Old Man Is Brought to the Emergency Department Due