A 52-year-old man from Delhi presents with a 6-month history of epigastric pain, heartburn, and nausea. Upper endoscopy reveals a 2 cm ulcer in the gastric antrum with surrounding erythema. Serum gastrin level is 850 pg/mL (normal <100). Gastric pH is 1.2. On further investigation, a 1.5 cm neuroendocrine tumor is found in the pancreatic head on CT imaging. Which of the following best explains the mechanism of acid hypersecretion in this patient?