A 20-year-old National Serviceman presented with six weeks of left anterior knee pain and swelling after training for his individual physical fitness test. The pain was predominantly over the lateral aspect of his patella and was exacerbated by running and squatting for prolonged periods. There was also occasional clicking in the knee and the sensation of instability.Physical examination revealed tenderness over the lateral joint line and lateral patellar edge. Although there was full range of movement of the knee, pain was elicited at extreme flexion. Patellar grind test and lateral McMurray's test were positive. What do the radiograph (Fig. 1) and subsequent magnetic resonance (MR) images (Fig. 2) of the knee show? What is the diagnosis?