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
Naltrexone is:- 
Select and explain. 
A. Mu receptor agonist, B. Delta receptor agonist, C. Kappa receptor agonist, D. Mu receptor antagonist
D: Mu receptor antagonist
Answer: D: Mu receptor antagonist 
Explanation: 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