An 12-year-old girl presents to your office with complaints of intermittent snapping and pain in her left knee that has worsened over the last several months. She denies any injuries or prior surgeries to this knee. She is a very active soccer player, and plays on several teams. On exam she lacks full extension by 10 degrees, has a negative ligamentous evaluation and has fullness to palpation about the lateral knee. She also endorses catching and clicking in her knee as she approaches full extension. Her radiographs are shown in Figure A. Which of the following is the most likely diagnosis?