A 12-year-old moderately obese boy presents with right groin pain and a limp. He states that the pain is worse with walking and relieved by rest. The pain begins in his right groin and radiates to his right knee. The pain began about 1 month ago without antecedent trauma and has progressively worsened. He has no pain in any other joints or extremities. He has no recent infections and reports no associated fevers, chills, or malaise. He participates in physical education at school but is otherwise not involved in sports. He has no recent travel or camping trips and lives in an urban area. He takes no medications. There is no family history of joint problems. On physical examination, he is afebrile. The right lower extremity rests in a slightly externally rotated position compared to the contralateral extremity, and he resists internal rotation. There is no leg length discrepancy. The neurologic and vascular exams are normal. Erythrocyte sedimentation rate and C-reactive protein are within normal limits.