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
Naltrexone is used in opioid dependence to - 
Select and explain. 
A. Prevent respiratory depression, B. Treat withdrawal symptoms, C. Prevent relapse, D. Treatment of opioid overdose
C: Prevent relapse
Answer: C: Prevent relapse 
Explanation: Naltrexone is a pure opioid antagonist. It is competitive antagonist at all opioid receptors. It can be used orally, to assist in the rehabilitation of ex-opioid abusers who are fully withdrawn (otherwise it will induce an acute withdrawal syndrome). A patient who then takes an opioid, fails to experience the kick or euphoria because of the opioid antagonistic actions of the Naltrexone.