This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Anaesthesia
Muscle Rigidity is caused by which agent 
Select and explain. 
A. Fentanyl, B. Halothane, C. Ketamine, D. Droperidol
A: Fentanyl
Answer: A: Fentanyl 
Explanation: A i.e. Fentanyl