A 5-day old infant presents to the ED in respiratory distress. Using the hyperoxia test to try to differentiate between a respiratory cause from a cardiac etiology, 100% O2 is administered to the infant. However, the respiratory distress is worsened. Which of the following is the likely cause of this patient’s dyspnea?

Dr. Anton Helman is an Emergency Physician at North York General in Toronto. He is an Assistant Professor at the University of Toronto, Division of Emergency Medicine and the Education Innovation Lead at the Schwartz-Reisman Emergency Medicine Instititute. He is the founder, editor-in-chief and host of Emergency Medicine Cases.