A 3-year-old girl with a trisomy 21 karyotype has had an intermittent, barking cough for the past 2 days. She has no history of respiratory distress, fever, or sick contacts. Physical examination reveals a low-grade fever, inspiratory stridor, and mild suprasternal retractions. Which of the following is the most likely diagnosis?
a) Influenza
b) Retropharyngeal abscess
c) Bacterial tracheitis
d) Croup