A 48-year-old woman from Mumbai with a 3-year history of refractory peptic ulcer disease is referred for investigation. She has failed two courses of triple therapy for H. pylori and remains symptomatic despite continuous omeprazole 40 mg daily. Repeat endoscopy shows a large (3 cm) duodenal ulcer with surrounding edema. H. pylori rapid urease test is negative. Serum gastrin level is 650 pg/mL (normal <100). What is the most appropriate next step in management?