This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Small & Large Intestine, Subject name: Surgery
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? 
Select and explain. 
A. Angiography, B. Upper GI endoscopy, C. Abdominal ultrasound, D. Sigmoidoscopy
D: Sigmoidoscopy
Answer: D: Sigmoidoscopy 
Explanation: 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.