Review Question - QID 109491

A 6-month-old male presents to the pediatrician for a well-child visit. His mother reports that the patient has just started eating solids and sleeps well throughout the night. She notes that she often puts the patient to sleep on his stomach because he seems to breathe more easily in that position. The patient’s mother has noticed that the patient’s breathing becomes more “strained” when lying on his back. She cannot remember when this problem began, but she believes it has gotten worse in recent weeks. The patient was born at 40 weeks gestation and has no other past medical history. The patient’s temperature is 98.0°F (36.7°C), blood pressure is 75/55 mmHg, pulse is 115/min, and respirations are 24/min. His oxygen saturation is 98% on room air. On physical exam, the patient appears comfortable and has inspiratory stridor that improves while leaning forward. His lungs are otherwise clear to auscultation bilaterally. Which of the following is the most likely cause of this patient’s respiratory symptoms?