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
Not true about nocturnal penile tumescence is - 
Select and explain. 
A. Totals about 100 min / night, B. Normal phenomenon, C. Occurs in NREM sleep, D. Can be used to distinguish between psychological or organic impotence
C: Occurs in NREM sleep
Answer: C: Occurs in NREM sleep 
Explanation: Nocturnal penile tumescence occurs in REM sleep. It is a normal phenomenon and occurs for 80-120 minutes per night. It can differentiate psychogenic impotence from organic impotence.