A 48-year-old woman presents with severe recurrent peptic ulcer located in the proximal jejunum. Five years previously she underwent parathyroidectomy for hypercalcemia. Her brother was previously diagnosed as having Zollinger-Ellison syndrome. To confirm the diagnosis of Zollinger-Ellison syndrome, blood should be tested for levels of which of the following?

Zollinger-Ellison syndrome is caused by secretion of excessive amounts of gastrin by islet cells of the pancreas (gastrinoma). It should always be thought of in patients with peptic ulcer disease, whose ulcers are severe, refractory to management, recurrent or located distally, beyond the first part of the duodenum. Gastrin levels in the blood are increased markedly and can be raised further by secretin injection (paradoxical response). The source of gastrin level in the blood may arise from hyperplasia, adenoma, or most commonly carcinoma of the islets. Most gastrinomas are sporadic, but 25% of patients have a family history of multiple endocrine neoplasia.