A 38-year-old woman with a 6-month history of recurrent epigastric pain and diarrhea is found to have a gastric ulcer on endoscopy. Fasting serum gastrin is 280 pg/mL (normal <100), and gastric pH is 2.1. Secretin stimulation test shows a gastrin rise of 180 pg/mL. What is the most appropriate next step in management?