A 22 year old male, Naveen , comes to the ER c/o a 3 day hx of right knee pain. The pain has become progressively worse to the point that he is unable to bear weight on it at all.
Examination: temperature 38.0. right knee flexed to 30 degrees. He will not allow you to flex or extend the knee. It is warm, and there is redness as well as tenderness around the knee.