This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Endocrinology, Subject name: Medicine
A 25 year old male presnents with weakness, occasional vomiting hypotension, skin and mucous membrane pigmentgation. The diagnosis can be best established by - 
Select and explain. 
A. Metyrapone test, B. Basal plasma Coisol levle, C. 24-hour urinary 17-Ketosteroid, D. ACTH stimulation test
D: ACTH stimulation test
Answer: D: ACTH stimulation test 
Explanation: after administration of ACTH , coisol level increases. This can be used as an index of the functional reserve of adrenal gland for production of coisol ( Harrison 17 pg 2252)