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
Addison's disease is due to: 
Select and explain. 
A. Chronic insufficiency of adrenal cortex, B. Chronic insufficiency of adrenal medulla, C. Insufficiency of pancreas, D. Hypofunction of thyroid gland
A: Chronic insufficiency of adrenal cortex
Answer: A: Chronic insufficiency of adrenal cortex 
Explanation: None