Q. A 3-year-old boy presents to the emergency department with a 2-day history of worsening barking cough, fever, and hoarse voice. His mother reports that he has been increasingly irritable and has difficulty breathing. On examination, the child's vital signs are as follows: temperature 38.2°C (100.8°F), heart rate 110 beats/min, respiratory rate 30 breaths/min, and blood pressure 95/60 mm Hg. He has inspiratory stridor and mild intercostal retractions. A frontal neck X-ray reveals a "steeple sign." Which of the following is the most likely diagnosis?