A 4-year-old boy presents with a limp for 2 days. He had a viral upper respiratory tract infection last week. He complains of pain in his left hip and refuses to bear weight. On examination, he is afebrile, and holds his hip in slight flexion and external rotation. There is pain on passive internal rotation. Inflammatory markers (CRP, WCC) are normal or only mildly elevated. What is the most likely diagnosis?