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: Pharmacology
Naloxone for opioid receptor acts as: 
Select and explain. 
A. Pure agonist, B. Pure antagonist, C. Paial agonist, D. Agonist antagonist
B: Pure antagonist
Answer: B: Pure antagonist 
Explanation: Naloxone is a pure antagonist and is preferred over older weak agonist-antagonist agents that had been used primarily as antagonists, eg, nalorphine and levallorphan. Ref: Katzung, 11th edition, Chapter 31.