You are caring for a 25-year-old otherwise healthy patient in the PACU who is currently breathing room air with an oxygen saturation of 85%. A blood gas study reveals an arterial PO2 of 55 mm Hg and an arterial PCO2 of 70 mm Hg. Which of the following is the most likely cause of this patient's hypoxemia?
A. Right-to-left intracardiac shunt
B. Hypoventilation
C. Atelectasis
D. Pulmonary embolism
E. Intrinsic lung disease