A 5-year-old boy presents with an acute onset of a limp and right groin pain for two days. He had an upper respiratory infection last week. He is afebrile at 37.5°C (99.5°F), is able to bear weight, and his labs show a white blood cell count of 11,000 cells/mm³ and an erythrocyte sedimentation rate of 18 mm/hr. Which diagnosis is most likely?