Ans- C Ventilation-perfusion inequality a. Although hypoventilation, incomplete oxygen diffusion, and pulmonary shunts all are causes of hypoxemia, the most common cause is ventilation-perfusion inequality. b. The mismatch of ventilation and blood flow occurs to some degree in the normal upright lung but may become extreme in the diseased lung. c. The three indices used to measure ventilation-perfusion inequality are alveolar arterial PO2 difference, physiologic shunt (venous admixture), and alveolar dead space. d. Elevated 2,3-diphosphoglycerate (2,3-DPG) levels shift the oxygen dissociation curve to the right and thereby augment tissue oxygenation. This elevation does not result in hypoxemia.