A 10-year-old boy complains of two days of worsening right knee pain. He has been unable to ambulate on the leg since waking up this morning. He denies any recent trauma to the leg. Physical exam is notable for focal tenderness over the distal femur without a palpable fluid collection. His ESR is 68 mm/hr (normal <15) and CRP is 14 mg/dL (normal <1). His temperature is currently 101.2 degrees F. Radiographs are provided in Figures A and B. An aspiration of the knee yielding 7 mL of straw colored fluid reveals 1700 nucleated cells per mL, and no organisms on gram stain. Which of the following is the most appropriate next step?