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
Most common cause of knock knee 
Select and explain. 
A. Osteo-arthritis, B. Rheumatoid arthritis, C. Idiopathic, D. Trauma
C: Idiopathic
Answer: C: Idiopathic 
Explanation: None