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
In addison's disease, most diagnostic test is - 
Select and explain. 
A. Serum Na+, K+, renin, B. S. Na+, K+, saline suppression, C. Serum creatinine\/urea ratio, D. ACTH stimulation test
D: ACTH stimulation test
Answer: D: ACTH stimulation test 
Explanation: to assess adrenal reserve capacity for steroid Production ( Harrison 17 pg 2264)