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
Opioid with rnonoarnine action is? 
Select and explain. 
A. Tramadol, B. Pentazocine, C. Pethidine, D. Meperidine
A: Tramadol
Answer: A: Tramadol 
Explanation: Tramadol REF: Principles and practice of palliative care and suppoive oncology By Ann M. Berger, John L. Shuster, Jamie H. Von Roenn page 51 "Tramadol is synthetic analog of codeine that binds to the MOR (Mu opioid receptor) and blocks reuptake of serotonin and nor epinephrine. As a result of monoamine action naloxone cannot completely reverse its respiratory depression"