Osteoarthritis

A 15-year-old boy comes to his general practitioner because of generalized right knee pain that has gradually worsened over the past five months. He does not recall any trauma or any inciting event. He says that the pain is accompanied by catching and clicking of the joint and is worsened by physical activity. He works on a farm and enjoys playing soccer with his siblings but has been unable to do so for the past month due to his symptoms. His knee appears swollen and is tender on palpation medial to the patella. Range of motion is limited and the knee clicks as it extends. The McMurray test is negative. Imaging cannot be obtained because the patient is unable to travel to an imaging center. Which of the following is the most likely diagnosis?