This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: C.N.S, Subject name: Pharmacology
[{'content': 'Pure opioid antagonist: \nSelect and explain. \nA. Naloxone, B. Buprenorphine, C. Pentazocine, D. Morphine', 'role': 'user'}, {'content': 'Answer: A \nExplanation: Ans. A. NaloxoneNaloxone, naltrexone and nalmefene are potent mu receptor antagonist with significant blocking action on kappa and delta. Naloxone is short acting and naltrexone is long acting. Naloxone is drug of choice for acute opioid poisoning. Naltrexone is used for maintenance therapy for opioid poisoning.', 'role': 'assistant'}]