Jane is 16 years old and is a keen cyclist and gymnast training several times a week. She comes to the GP surgery with her mother who is concerned about her right knee. It has been painful intermittently for the last 6 months and although the pain is mild they are concerned as to the duration of symptoms. On further questioning the pain is made worse by kneeling and when jumping and there is no knee swelling or other affected joints. Jane has no other past medical history and is feeling well otherwise.
On examination there is tenderness over the tibial tuberosity made worse when the knee is extended against resistance. There is no knee joint swelling, full range of movement in both knees and she has a normal gait.
What is the most likely diagnosis?