A 10-year-old male presents with refusal to bear weight on his right lower extremity. He reports falling off his bicycle 5 days prior. His right knee is warm to the touch and an effusion is noted. He has a temperature of 103.4°F and his laboratory values are remarkable for elevated ESR, CRP, and WBC. Radiographs of the knee are normal. A knee aspiration is performed. Aspiration reveals 1,000 WBC with 30% PMNs. What is the best next step?