A 9-mоnth-оld unvаccinаted infаnt presents with fever, rhinоrrhea, and cough for 2 days. The child now has tachypnea, nasal flaring, and retractions with shallow, rapid respirations. On auscultation, diffuse wheezes with a prolonged expiratory phase are heard, along with crackles and rhonchi. The infant appears lethargic and has circumoral cyanosis. A chest radiograph shows hyperinflation with mild interstitial infiltrates. Which of the following is the most likely diagnosis?