Which of the following will have the highest magnitude of the increased Alveolar aeriolar gradient at rest?

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.