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
The most common cause of euvolemic hyponatremia is which of the following? 
Select and explain. 
A. Hypothyroidism, B. Hypoadrenalism, C. Hypehyroidism, D. SIADH
D: SIADH
Answer: D: SIADH 
Explanation: The syndrome of inappropriate antidiuresis is the most common cause of euvolemic hyponatremia. Reference: Harrisons Principles of Internal Medicine, 18th Edition, Page 345