<s>[INST] I was walking up stairs and all of a sudden I felt or heard a popand my leg gave out. I cannot put weight on it, it will not hold me up. I was treated Monday for fluid on knee and weakness, probably caused by irritation from a small tear in meniscus. Had a shot of cortizone, and by Tuesday night knee was much better. This came out of nowhere. What could it be? There is no pain until I try to put weight on it YYYY@YYYY [/INST] Hello, Consult an orthopaedic and get evaluated.  An MRI scan is required for further assessment. Hope I have answered your query. Let me know if I can assist you further. Take care Regards, Dr Shinas Hussain, General & Family Physician </s>