A previously healthy 42-year-old man comes to the physician because of 2 days of right knee pain and inability to extend the knee. The symptoms began when he was getting up from a low chair. His temperature is normal. Examination of the knee shows tenderness to palpation along the medial joint line and joint effusion. Diagnosis?
A) Patellar tendonitis
B) Quadriceps tendon rupture
C) Medial collateral ligament injury
D) Patellar dislocation