A 4-year-year-old boy presents with a limp and refusal to weight bear on his right leg, starting acutely yesterday morning. He had symptoms of an upper respiratory tract infection last week but is afebrile today. Examination reveals limited and painful range of motion at the right hip, particularly internal rotation. X-rays of the pelvis and hips are normal. Inflammatory markers (CRP, ESR) and WCC are normal or only mildly elevated. What is the most likely diagnosis?