A 2 yeаr оld with а histоry оf extreme premаturity, severe bronchopulmonary dysplasia (BPD), and tracheostomy with chronic ventilator support presents with acute hypoxemia and increased work of breathing. Parents report thick secretions over the past 24 hours. On exam, the child is tachycardic and has decreased breath sounds. Despite suctioning, oxygen saturations remain low. Which of the following should be highest on your differential diagnosis?