This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Muscle Relaxant, Subject name: Anaesthesia
Muscle rigidity by fentanyl is due to which receptor: 
Select and explain. 
A. Meu (m), B. Kappa (k), C. Delta (d), D. Sigma (s)
A: Meu (m)
Answer: A: Meu (m) 
Explanation: Ans. (a) Meu (m)Ref.: Millers Anaesthesia 7th ed. / 914Opioid receptors are of four variety - mu (m), Kappa (k), delta (d) & sigma (s)ReceptorClinical effectAgonist (m2)Muscle rigidityPhysical dependenceSupraspinal analgesiaRespiratory depressionFentanylMorphineb- EndorphinMu (m) (m2)Kappa (k)SedationSpinal analgesiaMorphineNalbuphineDynorphin (endogenous opioid)Butorphanol oxycodoneDelta (d)AnalgesiaBehavioralEpileptogenicLeu - Enkephalinb- EndorphinSigma (s)DysphoriaHallucinationRespiratory stimulationPentazocineNalorphineKetamine