A 4-year-old boy is brought to the emergency department by his mother. He has had 24 hours of fever and a limp, and now refuses to weight-bear on his left leg. His temperature is 39.0°C, pulse 138/min and capillary refill time is 2 seconds. He lies with the left hip flexed and externally rotated, and cries when the hip is moved even slightly. There is no history of trauma. White cell count is 18.4 × 10⁹/L (4.0–11.0) and C-reactive protein 96 mg/L (below 10). Blood cultures have been taken.

What is the most appropriate management?