Meckel Diverticulum with GI Bleeding

A 22-month-old previously well boy presents to the emergency department with 12 hours of passing large amounts of maroon to brick-red blood per rectum without preceding abdominal pain or vomiting. He is now pale, lethargic, tachycardic (HR 160/min), with cool peripheries and capillary refill of 3 seconds. Abdominal examination is soft and non-tender without a palpable mass. Nasogastric aspiration shows bile-stained clear fluid. A Technetium-99m pertechnetate scan shows an abnormal focus of tracer uptake in the right lower quadrant appearing simultaneously with the gastric mucosa. The structure marked **B** in the diagram is responsible for this presentation. What is the underlying pathophysiological mechanism of bleeding in this condition?