Ischemic Colitis

A 72-year-old male with coronary artery disease, atrial fibrillation on warfarin, and peripheral arterial disease presents with sudden-onset left-sided cramping abdominal pain and bright red blood per rectum following a recent hypotensive episode (SBP 68 mmHg). CT abdomen/pelvis shows segmental wall thickening of the descending colon and splenic flexure with thumbprinting and patent mesenteric vessels. Sigmoidoscopy is performed. The structure marked **A** in the diagram—watershed segmental pale mucosa with linear ulcers—is characteristic of which pathophysiologic mechanism in this patient?