A 41-year-old woman is known to have multiple endocrine neoplasia syndrome. She has multiple family members who have had adenoma tumors removed from the parathyroid, pancreas, and/or pituitary glands. She has severe diarrhea associated with low gastric acid secretion and a normal gastrin level. Which of the following serum assays would be best to evaluate the possible cause of the diarrhea?
Glucagon Vasoactive intestinal peptide (VIP) Cholecystokinin Serotonin