You are answering a USMLE-style medical question. Question: A 35-year-old man returns to the clinic to follow up for his chronic stomach pain. At the last visit a few months ago, he explained that he had been experiencing discomfort in his upper abdomen for awhile. He had never vomited up any blood and had not had any substantial weight loss. He did not take any medications, did not smoke, and had no family history of gastric cancer. At that time, the doctor empirically started him on a proton pump inhibitor (PPI). Today, despite the PPI, the patient says he is still experiencing discomfort. Hearing this, the doctor decides to order a urease breath test. What is the most likely cause of this patient's chronic stomach pain? Options: A) Gastroesophgeal sphincter dysfunction B) Nonsteroidal anti-inflammatory drugs C) Heliobacter pylori infection D) Excessive gastrin Reply with ONLY the letter of the correct answer (A, B, C, or D). No explanation.