A 49-year-old man comes to the physician because of a history of recurrent attacks of drowsiness, sweating, and palpitations. His blood glucose level during one of the episodes is 52 mg/dL. Recovery occurs in response to intravenous 10% dextrose C-peptide levels are below the normal range. 
Which of the following is the most likely diagnosis?
Factitious insulin use
Gastrinoma
Glucagonoma
Insulinoma