Review Question - QID 109780

A 7-year-old boy presents to the pediatric emergency department for left knee pain. He fell while riding his skateboard 3 days ago and developed knee pain over the subsequent days. He is now complaining of worsening pain and is unable to walk. He has no significant medical history. His temperature is 102.2°F (39.0°C), blood pressure is 95/48 mmHg, pulse is 120/min, respirations are 20/min, and oxygen saturation is 98% on room air. On exam, he is in distress due to pain. The left knee is erythematous and warm with a large effusion. Passive flexion and extension of the knee is limited by pain. The patient refuses to bear weight on the left leg or to walk. Which of the following is the most appropriate initial step in management?