A 55-year-old man who is extremely obese reports weakness, sweating, tachycardia, confusion, and headache whenever he fasts for more than a few hours. He has prompt relief of symptoms when he eats. These symptoms are most suggestive of which of the following disorders?
a. Diabetes mellitus
c. Zollinger-Ellison syndrome
d. Carcinoid syndrome
e. Multiple endocrine neoplasia, type II