A 78-year-old male with severe aortic stenosis (valve area 0.7 cm²) presents with recurrent painless bright-red per-rectal bleeding. Urgent colonoscopy reveals three discrete vascular lesions in the caecum and ascending colon. The structure marked **A** in the diagram shows the characteristic endoscopic appearance of colonic angiodysplasia: a cherry-red flat lesion 6 mm in diameter with a distinctive spidery branching pattern of submucosal vessels. An adherent clot overlies the largest lesion. Which of the following best explains the pathophysiology linking his aortic stenosis to the development of these lesions marked **A**?