Three days after undergoing an operation for an abdominal aortic aneurysm, a patient has moderate fever, abdominal pain, and rectal bleeding. What is the most helpful investigation?

In a patient with abdominal aortic aneurysm resection, the most worrisome complication is inadequate blood supply to the sigmoid colon through the marginal artery. Sigmoid ischemia should be ruled out by sigmoidoscopy. In the clinical picture described, sigmoidoscopy should be the most important test.