Neuroendocrine tumors of the gastrointestinal system: Pathology review

A 47-year-old woman comes to the physician due to midline abdominal pain that has been ongoing for several months. The pain is described as a burning sensation and worsens after meals. As a result, the patient has been eating less and has experienced a 5 kg (11 lb) weight loss. She has a history of peptic ulcer disease, which was diagnosed last year. She was prescribed omeprazole but has not noticed an improvement in symptoms after initiating therapy. Medical history is also notable for low back pain. Vitals are within normal limits. Physical examination reveals mild tenderness to palpation of the epigastric region. An endoscopy is performed and shows multiple ulcers in the stomach and duodenum. A urea breath test is negative. Administration of intravenous secretin is followed by an increase in serum gastrin levels by 200 pg/mL. Which of the following is the most likely cause of this patient’s symptoms?