This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Anesthesia, Subject name: Pharmacology
The drug for OPD analgesia is: 
Select and explain. 
A. Morphine, B. Pethidine, C. Fentanyl, D. Alfentanil
D: Alfentanil
Answer: D: Alfentanil 
Explanation: Alfentanil is a potent but sho-acting synthetic opioid analgesic drug, used for anaesthesia in surgery. It is an analogue of fentanyl with around 1/4 to 1/10 the potency of fentanyl and around 1/3 of the duration of action, but with an onset of effects 4x faster than fentanyl.