This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Medicine
Which of the following is not used in diagnosis of insulinoma - 
Select and explain. 
A. 72 hours fasting blood glucose levels, B. C peptide levels, C. Insulin/glucose ratio, D. D-xylose test
D: D-xylose test
Answer: D: D-xylose test 
Explanation: None