A 2-year-old boy presents to the emergency department with a 3-day history of barking cough and inspiratory stridor. A lateral neck radiograph is obtained. The structure marked **D** (trachea) appears normal in caliber. However, an anteroposterior (AP) view of the neck shows subglottic narrowing with a characteristic "steeple sign." Which of the following is the most likely diagnosis?

Explanation

The steeple sign is a characteristic radiographic finding on anteroposterior (AP) neck films that represents subglottic narrowing of the trachea. This finding is pathognomonic for croup (laryngotracheobronchitis), a viral infection typically caused by parainfluenza virus in children aged 6 months to 3 years. The clinical presentation of barking cough, inspiratory stridor, and hoarse voice in a toddler is classic for croup. The lateral neck film may appear relatively normal (as in this case, with structure **D** showing normal caliber), but the AP view reveals the characteristic narrowing. Croup is managed with nebulized epinephrine and dexamethasone. The normal appearance of the trachea on the lateral view helps exclude other serious conditions such as epiglottitis or bacterial tracheitis, which would show different radiographic and clinical findings.