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nbme24/Block 2/Question#19

A 70-year-old man comes to the physician because of ...

Alveolar-arterial Po 2 difference

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submitted by neonem(257),

This patient has pulmonary fibrosis, which causes a restrictive (not obstructive)-type disease. Since there was no occupational exposure, I'm assuming this is idiopathic pulmonary fibrosis. This causes thickened alveolar membranes, limiting gas diffusion. Therefore, eventually O2 won't be able to diffuse quickly enough into the blood across the alveolar-arterial membrane, resulting in a larger A-a difference. (I think there's normally a small A-a gradient, from 2-14 mm Hg, but when this gets too big, you get hypoxic)

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