All are decreased in infiltrative lung disease, except:

Answer is B (Alveolar aerial difference in PaO2) If the gas exchange at alveolar capillary level occurs normally, the alveolar aerial difference in PaO2 is not elevated. In infiltrative lung disease the alveolar capillary bed is damaged and there is a decrease in diffusion capacity. This results in an increase in alveolar aerial differences in Pa O2.