A 2-year-old boy is brought to the pediatrician by his mother for concern about clear, watery nasal discharge and a "barking" cough for the past 48 hours. She cannot identify any precipitating factors. The child never had symptoms like this and has no significant past medical history. He has not had any recent travel and has received all age-appropriate immunizations. He is at the 42nd percentile for height and 53rd percentile for weight, which is consistent with his last visit. The patient is febrile but otherwise vitals are within normal limits. Physical examination reveals clear rhinorrhea and a high-pitched noise coming from the child as he breathes.

Which of the following is the most likely etiology for this child’s illness?

• A) A double-stranded DNA virus
• B) A gram-negative rod
• C) A gram-positive coccus
• D) A negative-sense, single-stranded RNA virus
• E) A positive-sense, double-stranded RNA virus