This patient's presentation is consistent with sigmoid volvulus, a condition that occurs most commonly in debilitated or institutionalized elderly adults with a history of constipation.

Sigmoid volvulus occurs when a segment of sigmoid colon twists on its mesentery, forming a closed-loop obstruction.  With continued gas formation by intraluminal bacteria, the lumen of the obstructed bowel loop gradually expands.  This leads to slowly progressive (eg, over 2 days) abdominal distension and discomfort frequently accompanied by loss of appetite, nausea, vomiting, and/or obstipation.  As in this patient, abdominal x-ray often reveals a dilated, inverted, U-shaped loop of colon with the appearance of a "coffee bean."

Increased length (causing redundancy) of the sigmoid colon compared to its mesentery increases the risk that the sigmoid colon will twist around its mesentery.  Because chronic constipation, which causes chronic fecal overloading, can lead to dilation and elongation of the sigmoid colon, it is considered a risk factor for sigmoid volvulus.  Colonic hypomotility (eg, neurogenic bowel in neurologic disorders, diabetic enteropathy) can contribute to constipation and increase the risk of volvulus (Choice B).

(Choice C)  Electrolyte abnormalities (eg, hypokalemia and hypophosphatemia, which cause muscle dysfunction) can cause paralytic ileus.  Ileus typically appears on x-ray as generalized, uniform distension of the large and small bowel.  In contrast, this patient's single dilated loop of bowel indicates a closed-loop obstruction (eg, sigmoid volvulus).

(Choice D)  The use of nonsteroidal anti-inflammatory drugs, which is common in the elderly population, can be complicated by gastroduodenal ulceration and perforation.  However, gastrointestinal tract perforation classically appears on x-ray as free air under the diaphragm.

(Choice E)  Antibiotic therapy may be complicated by Clostridioides (formerly Clostridium) difficile colitis, which can lead to toxic megacolon with colonic dilation.  However, x-ray typically reveals diffuse dilation of the colon within its normal anatomic arrangement, rather than in a loop, and patients typically appear toxic (eg, fever, altered sensorium) and may have severe bloody diarrhea, unlike in this patient.

Educational objective:
Sigmoid volvulus occurs when a segment of sigmoid colon twists on its mesentery, forming a closed-loop obstruction that often appears on abdominal x-ray as a dilated, inverted, U-shaped loop ("coffee bean" sign).  Chronic constipation and colonic dysmotility are risk factors.