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 monoamine action is? 
Select and explain. 
A. Tramadol, B. Pentazocine, C. Pethidine, D. Meperidine
A: Tramadol
Answer: A: Tramadol 
Explanation: Ans. is 'a' i.e., Tramadol Unlike other opioids, tramadol inhibits reuptake of NA and 5-HT and thus activates monoaminergic spinal inhibition of pain --> action can be abolished by 5-HT3 antagonist (ondansetron).