A previously healthy 2-year-old black child has developed a chronic cough during the previous 6 weeks. He has been seen in different emergency rooms on two occasions during this period and has been placed on antibiotics for pneumonia. Upon auscultation, you hear normal breath sounds on the left. On the right side, you hear decreased air movement during inspiration but no air movement upon expiration. Inspiratory (A) and expiratory (B) radiographs of the chest are shown below. Which of the following is the most appropriate next step in making the diagnosis in this patient?