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: Orthopaedics
[{'content': 'Most common cause of knock knee \nSelect and explain. \nA. Osteo-arthritis, B. Rheumatoid arthritis, C. Idiopathic, D. Trauma', 'role': 'user'}, {'content': 'Answer: C \nExplanation: None', 'role': 'assistant'}]