A 48-year-old man comes to the office because of a 3-month history of burning epigastric pain that eases shortly after eating but returns 2 to 3 hours later, often waking him around 2 AM. He emigrated from Vietnam 15 years ago and takes no medications, including no proton pump inhibitors or NSAIDs. Epigastric tenderness is present on deep palpation. A urea breath test is positive. Fasting serum gastrin is 195 pg/mL (reference range: <100 pg/mL). Upper endoscopy reveals a clean-based 8-mm ulcer in the duodenal bulb.
Which of the following mechanisms most directly explains the elevated fasting gastrin level in this patient?