A patient is admitted to the hospital because of hypoxemia.While the patient breathes ambient air,the arterial partial pressure of oxygen (Pao2)is 50 mm Hg with an oxygen saturation of 82%.When the patient breathes 100% oxygen through a non-rebreathing mask,the Pao2 is 83 mm Hg with an oxygen saturation of 92%.The cause of this patient's hypoxemia is probably:
A)A physiological shunt.
B)Low V̇/Q̇.
C)Hypoventilation.
D)Decreased inspired oxygen.