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: Psychiatry
A 50 year old male presents with a 3 year history of irritability, low mood, lack of interest in surroundings and general dissatisfaction with everything. There is no significant disruption in his sleep or appetite. He is likely to be suffering from- 
Select and explain. 
A. Major depression, B. No psychiatric disorder, C. Dysthymia, D. Chronic fatigue syndrome
C: Dysthymia
Answer: C: Dysthymia 
Explanation: Irritability, low mood (low self esteem, pessimism, hopelessness) and lack of interest for 3 years suggest the diagnosis of dysthymia (for details, see the table of diagnostic criteria in text).