A 6-month-old infant presents with a 3-day history of coryza, cough, and increasing respiratory distress with audible wheeze. On examination, temperature is 37.8°C, respiratory rate is 60/min with subcostal retractions, and widespread expiratory wheezes and crackles are heard on auscultation. Oxygen saturation is 91% in air. What is the most likely diagnosis?