This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: CNS, Subject name: Pharmacology
[{'content': "An analgesic 'X' acts through opioid as well as additional spinal monoaminergic mechanisms. Which of the following can be 'X'? \nSelect and explain. \nA. Tramadol, B. Etioheptazine, C. Dextropropoxyphene, D. Alfentanil", 'role': 'user'}, {'content': 'Answer: A \nExplanation: -Tramadol and tapendadol have additional analgesic mechanisms apa from activation of opioid receptors. -These drugs increase release of serotonin in spinal cord and strengthen the descending inhibitory pathway.', 'role': 'assistant'}]