A 55-year-old man presents with upper gastrointestinal bleeding and is found to have multiple ulcers in the duodenum. Workup finds his serum gastrin levels to be markedly elevated, and an abdominal CT scan finds a large mass in the tail of the pancreas. A biopsy from this pancreatic mass finds an islet-cell adenoma that secretes gastrin. Which of the following is the best diagnosis