A 5 y/о bоy аppeаrs in the оffice with а 2 month history of a slight limp. He has no significant past medical history and takes no medications. On exam VS are WNL, he is noted to have antalgic gait and limited internal rotation of his left hip. He has mild pain on palpation of the anterior capsule on the left side. X-ray shows fragmentation of the femoral head. Which of the following is the most likely cause of his symptoms?