A 3-year-old boy is brought to the GP with a 1-day history of limping. He has no history of trauma. He recently recovered from a viral upper respiratory tract infection. On examination, he is afebrile and well-appearing. He has limited range of movement in his right hip, especially internal rotation and abduction. Blood tests show: WCC 9.0 x10^9/L, CRP 5 mg/L. Hip X-ray is normal. What is the most likely diagnosis?