Ortho Dx: "Popping" and pain in an adolescent's knee

A 13-year-old girl presents with a 4-month history of anterior lateral left knee pain. She reports episodic "popping" and pain in the knee while running. She denies any swelling or feeling of instability in the knee. Examination reveals pain over the lateral joint line, which is made worse with knee extension and valgus stress. Bilateral standing ...