The correct answer is E. The diagnosis is ischemic colitis, a well-recognized complication of abdominal aortic aneurysm (AAA) repair, open or endovascular. Often the origin of the inferior mesenteric artery is covered by the aortic graft, and patients who do not have adequate collateralization via the marginal artery of Drummund are at risk for ischemia of the descending and sigmoid colon. The rectum is not involved because it has a different arterial blood supply.

Bloody diarrhea and leukocytosis in the postoperative period are essentially diagnostic of this condition, but the gold standard remains endoscopic visualization of a cyanotic or shedding mucosa, via either colonoscopy or flexible sigmoidoscopy. Management almost exclusively requires a colonic resection with colostomy, and lack of prompt intervention could result in sepsis and death.

CT scan of the abdomen and pelvis (choice A) may show a thickened colonic wall consistent with colitis or, in more severe cases, pneumatosis intestinalis. CT scan will more often be negative and provide no diagnosis. It also will not provide information to differentiate the etiology (i.e., ischemic versus infectious), and postoperative intra-abdominal fluid and edema may obscure the images. Moreover, the use of IV contrast, needed for the abdominal CT scan, is not appropriate, given this patient's increasing creatinine.

Repeating the laboratory tests (choice B) is often a good idea to confirm abnormal findings, but it is not the best option, especially in a clinically deteriorating patient. A diagnosis needs to be made immediately.

Although diarrhea and an increasing leukocytosis are worrisome in Clostridium difficile infections (choice C), the diarrhea is typically watery and foul-smelling, not bloody. Cytotoxin assay is the gold standard for the diagnosis of pseudomembranous colitis. The clinical presentation is worrisome for an ischemic complication from the AAA repair and not C. difficile, and this route should be investigated first.

The decrease in hemoglobin and bloody stools are to be taken into account. Even though the decrease in hemoglobin is mild and this patient's AAA puts him at risk for other atherosclerotic conditions, he is not likely to need a transfusion yet. If the patient's condition warrants transfusion, packed red blood cells, not whole blood, is administered (choice D).