Gastritis

A 55-year old woman comes to the emergency department because of light headedness, fatigue, and a racing heart for 6 hours. She tells you that she has also had very dark stools in the last day. Her medical history includes hyperlipidemia, and daily cigarette smoking. Her temperature is 36.8°C (98°F), pulse is 110/min, respirations are 23/min, and blood pressure is 90/65 mm Hg. Abdominal examination is noncontributory. A biopsy from upper gastrointestinal endoscopy shows partial replacement of the gastric mucosal epithelium by intestinal metaplasia in the antrum. Which of the following most likely explains this patient’s problem?