A 78-year-old man underwent surgical repair of an abdominal aoic aneurysm. Postoperatively he developed left lower quadrant pain along with urgency to defecate and had frequent episodes of passage of bright red blood per rectum. He has a history of prostate cancer for which he had received radiation therapy several years ago. Colonoscopy findings of the patient are shown below.X ray abdomen was also done. What is the most likely cause of his symptoms?