A 15-year-old male presents with a 2-month history of progressive pain and swelling in his right distal femur. He reports that the pain is worse at night and is not relieved by rest. On physical examination, there is a firm, tender mass palpable over the distal femur. X-ray reveals a destructive lesion with a “sunburst” appearance and periosteal reaction. What is the most likely diagnosis?
