Q. A 44-year-old woman comes to the clinic because of a 2 month history of episodic burning abdominal pain that worsens after eating meals. It is in the midepigastric region. She has a history of chronic headaches managed with ibuprofen. Her vital signs are normal. She has smoked 2 packs of cigarettes a day for the past 20 years. On examination, she does not have any significant abdominal pain with palpation and no masses are identified. The urea breath test is negative. Upper endoscopy shows an ulcer in the stomach antrum. What is the most likely mechanism that caused this ulcer?