A 4-year-old girl is brought to the emergency department after being entirely well this morning. Over the past 4 hours she developed a high fever and severe sore throat. She has not received any of her recommended childhood vaccines. On arrival she is sitting bolt upright on the stretcher, hands pressed into the mattress, leaning forward with her neck extended and her mouth open. Saliva collects at the corners of her mouth; she refuses to swallow. She answers questions in a few thick, muffled words and will not lie down when asked. She has not been coughing. Temperature is 39.8°C, heart rate 138/min, respiratory rate 32/min, and oxygen saturation 94% on room air. She appears toxic and frightened. Inspiratory stridor is audible at rest. A lateral neck radiograph shows a rounded, thumb-shaped soft-tissue density at the level of the hypopharynx.
Which of the following is the most appropriate next step in management?