This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: General anaesthesia, Subject name: Anaesthesia
The most potent synthetic opioid is 
Select and explain. 
A. Remifentanil, B. Sufentanil, C. Alfentanil, D. Fentanyl
B: Sufentanil
Answer: B: Sufentanil 
Explanation: Sufentanil has a high affinity for the mu receptor, higher than that of any other opioid.