A 67-year-old man with a history of uncontrolled hypertension, heavy cigarette smoking, and chronic renal insufficiency undergoes an open repair of a leaking abdominal aortic aneurysm (AAA). On postoperative day 2, he has passage of bright red blood per rectum associated with increased abdominal pain and leukocytosis. On examination, he has tenderness in the left lower quadrant without peritoneal signs. His vital signs show a blood pressure of 90/50 mmHg and a heart rate of 110 beats per minute. You initiate intravenous (IV) fluid resuscitation and obtain computed tomography (CT) of the abdomen and pelvis, which revealed a thickened sigmoid colon with no pneumoperitoneum. You initiate bowel rest and IV antibiotics. The best next step is