This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: COPD and Asthma, Subject name: Medicine
Which of the following will have the highest magnitude of the increased Alveolar aeriolar gradient at rest? 
Select and explain. 
A. Interstitial lung disease, B. Massive Pulmonary embolism, C. Acute severe asthma, D. FB leading to upper airway obstruction
B: Massive Pulmonary embolism
Answer: B: Massive Pulmonary embolism 
Explanation: Normal alveolar aeriolar gradient is 5-15 mm Hg. In interstitial lung disease, diffusion effect is present but PO2 can be normal and decreases in exercise. Hence in ILD A-a gradient is normal at rest and increases in exercise Massive pulmonary embolism, the increased dead space leads to very low PO2 and an increase of A-a gradient. Choice C and D are easily ruled out as both leads to hypoventilation leading to normal A-a gradient.