A 48-year-old woman from Mumbai presents with a 3-month history of severe diarrhea, weight loss, and abdominal cramping. She has a past medical history of rheumatoid arthritis treated with NSAIDs for 5 years. Endoscopy shows multiple gastric ulcers with a clean base. Serum gastrin is 95 pg/mL (normal <100), and gastric pH is 3.8. Which of the following mechanisms best explains the reduced gastric acid secretion in this patient?