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
Increased Alveolar arterial difference in PaO2 is seen in all except? 
Select and explain. 
A. COPD, B. Pulmonary Edema, C. GBS, D. Interstitial Lung Disease
C: GBS
Answer: C: GBS 
Explanation: GBS is extra-parenchymal restrictive lung disease, so P(A - a)O2 is normal.