A 32-year-old male patient has experienced intermittent epigastric pain for 3 years, with occasional acid regurgitation and belching, worsening after catching cold. Pain intensified over the past 3 days. Today, he suddenly vomited 500ml of coffee-ground material, accompanied by dizziness, palpitations, and sweating. Abdominal pain relieved after bleeding. The most likely cause of bleeding is: