Solitary Rectal Ulcer Syndrome

A 31-year-old woman presents with an 8-month history of rectal bleeding, mucus passage, and tenesmus. She has chronic constipation with excessive straining and admits to digital disimpaction. Sigmoidoscopy reveals a solitary, shallow, well-circumscribed ulcer with fibrinous slough on the anterior rectal wall 7 cm above the dentate line, surrounded by hyperemic, polypoid, thickened mucosa. Biopsy demonstrates fibromuscular obliteration of the lamina propria with splayed muscularis mucosa and regenerative crypt distortion, without granulomas or crypt abscesses. The condition marked **B** in the diagram is characterized by which of the following pathophysiologic mechanisms?