A 48-year-old woman from Mumbai presents with severe epigastric pain, diarrhea, and weight loss over 6 months. Endoscopy shows multiple large ulcers in the gastric antrum and duodenum, with unusually severe esophageal erosions. Fasting serum gastrin is 580 pg/mL (normal <100 pg/mL), and gastric pH is 1.2. Secretin stimulation test shows a rise in gastrin from 580 to 720 pg/mL. Which of the following best explains the pathophysiology of acid hypersecretion in this patient?