This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Medicine
All are decreased in infiltrative lung disease, except: 
Select and explain. 
A. Vital capacity, B. Alveolar aerial difference in PaO2, C. Total lung capacity, D. Lung compliance
B: Alveolar aerial difference in PaO2
Answer: B: Alveolar aerial difference in PaO2 
Explanation: 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.