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
Muscular timidity caused by opioids is due to the agonist effect on which receptor 
Select and explain. 
A. Mu, B. Kappa, C. Delta, D. Sigma
A: Mu
Answer: A: Mu 
Explanation: Refer Miller 7/ep781 Truncal rigidity caused by highly lipid soluble opioids like Fentanyl is supraspinal in origin. It is mainly caused by stimulation of mu receptors whereas kappa and delta receptors tend to reduce the rigidity