A 3-year-old girl presents with a limp and refusal to weight bear on her right leg, which started acutely this morning. She had a mild upper respiratory tract infection last week. On examination, she is afebrile, and her right hip has limited and painful abduction and internal rotation. X-ray of the pelvis and hips is normal. Inflammatory markers (CRP, WCC) are normal or only mildly elevated. What is the most likely diagnosis?