A 3-year-old girl presents with a limp and refusal to weight bear on her right leg, starting acutely this morning. She had a viral URTI last week. On examination, she is afebrile and systemically well. There is pain on passive internal rotation of the right hip, but no swelling, warmth, or erythema. Blood tests including FBC, ESR, and CRP are normal. X-ray of the pelvis and hips is normal. What is the most likely diagnosis?