This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Mood disorders, Subject name: Psychiatry
Most common psychiatric illness 
Select and explain. 
A. Depression, B. Bipolar, C. Mania, D. Cyclothymia
A: Depression
Answer: A: Depression 
Explanation: Mood disorders are common. In the most recent surveys, major depressive disorder has the highest lifetime prevalence (almost 17 percent) of any psychiatric disorder. Reference: Synopsis of Psychiatry, page 348.