A 4-year-old boy is brought to the emergency department by his parents with a 1-day history of fever and refusal to walk. He has been irritable, and they noticed he winces when his left leg is moved or touched. There is no history of trauma. Upon examination, his temperature is 38.9°C, his heart rate is 130 beats per minute, and he appears visibly distressed. His left hip is flexed, abducted, and externally rotated, and he cries when any attempt is made to move it. The joint appears swollen and warm.