This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Neuromuscular Blocker, Subject name: Anaesthesia
The severe bronchospasm associated with rapacuronium, the cause is 
Select and explain. 
A. Allergic reaction, B. Massive histamine release, C. Unopposed action M3 muscarinic receptors in the bronchus., D. M2 muscarinic receptor activation
C: Unopposed action M3 muscarinic receptors in the bronchus.
Answer: C: Unopposed action M3 muscarinic receptors in the bronchus. 
Explanation: Rapacuronium has effect on M2 and M3 muscarinic receptors in the bronchus. Activation of the postsynaptic M3 receptor by acetylcholine produces bronchoconstriction and the effect is terminated by presynooptic M2 receptors. Repacuronium blocks M2 receptor and has little effect on M3 so bronchoconstriction.