You are answering a USMLE-style medical question. Question: An otherwise healthy 50-year-old man comes to the physician because of a 6-month history of increasingly frequent episodes of upper abdominal pain, nausea, vomiting, and diarrhea. He has had a 3.2-kg (7-lb) weight loss during this time. Physical examination shows bilateral pitting pedal edema. An endoscopy shows prominent rugae in the gastric fundus. Biopsy shows parietal cell atrophy. Which of the following is the most likely underlying cause? Options: A) Serotonin-secreting gastric tumor B) Proliferation of gastric mucus-producing cells C) Excessive somatostatin secretion D) Ectopic secretion of gastrin Reply with ONLY the letter of the correct answer (A, B, C, or D). No explanation.