mrcem2-1770

A 6-month-old baby boy is brought to the Emergency Department with a 3-day history of cold symptoms. He is coughing frequently, and his mother has noticed that he is wheezing. He is now having difficulty with feeding and appears fatigued. On examination, his respiratory rate is 60, and you can see flaring of the nostrils and chest wall retractions. Chest examination reveals bilateral fine crackles and high-pitched expiratory wheezing in both lung fields.

What is the SINGLE most likely diagnosis?