A 3-year-old boy is brought to the emergency department because he has stopped walking and cries when lifted. He has a temperature of 38.8°C, a heart rate of 132/min and holds the left hip flexed and externally rotated. The white cell count is 17.2 × 10^9/L (5.0–15.0) and C-reactive protein is 112 mg/L (<5); ultrasound shows a hip effusion. Blood cultures have already been collected, and he is haemodynamically stable. Which one of the following is the most appropriate next step?