This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Opioids and Alcohols, Subject name: Pharmacology
[{'content': 'Naltrexone is:- \nSelect and explain. \nA. Mu receptor agonist, B. Delta receptor agonist, C. Kappa receptor agonist, D. Mu receptor antagonist', 'role': 'user'}, {'content': 'Answer: D \nExplanation: Naloxone, naltrexone and nalmefene are potent mu receptor antagonists with significant blocking action at kappa and delta receptors also. Naloxone is given parenterally (ineffective orally) and is a very sho acting drug. Nalmefene is also given parenterally but has a longer half life. Naltrexone is long acting orally effective opioid antagonist. Naloxegol-Naloxone conjugated to PEG polymer', 'role': 'assistant'}]