A 65-year-old woman with a history of chronic constipation is transferred from a nursing home because of abdominal pain and marked abdominal distention. On examination, her abdomen is found to be distended and tender in the LLQ. What is the most likely diagnosis?

Volvulus of the sigmoid (secondary type) is common in elderly patients who are chronically constipated. Redundancy of the sigmoid and a narrow mesenteric attachment predispose for the twisting. In the large bowel, the sigmoid is the most common site. Abdominal distention and tenderness are the common presenting symptoms. Volvulus of the sigmoid colon can usually be detected on a supine and erect abdominal x-ray. Sigmoidoscopy and contrast barium studies may be helpful to differentiate carcinoma from volvulus (Figure below).Volvulus of the sigmoid colon. Barium enema taken with the patient in the supine position. Note the massively dilated sigmoid colon. The distinct vertical cease, which represents juxtaposition of adjacent walls of the dilated loop, points toward the site of torsion. The barium column resembles a "bird's beak" or "ace of spades" because of the way in which the lumen tapers toward the volvulus.