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
[{'content': 'Muscle Rigidity is caused by which agent \nSelect and explain. \nA. Fentanyl, B. Halothane, C. Ketamine, D. Droperidol', 'role': 'user'}, {'content': 'Answer: A \nExplanation: A i.e. Fentanyl', 'role': 'assistant'}]