Alveolar-aerial tension gradient increases in all except?

Alveolar aeriolar gradient = pAO2- pAO2 Choices A, C, D will alter only pa02 leading to an increase in gradient. Higher the gradient more severe is the disease and would require ventilator suppo. Causes of increase alveolar aeriolar mismatch are: 1. V/Q Mismatch (e.g. PNA, CHF, ARDS, atelectasis) 2. Shunt (e.g. PFO, ASD, PE, pulmonary AVMs) 3. Alveolar Hypoventilation (e.g. interstitial lung disease, environmental lung disease) Remember that hypoventilation due to CNS diseases or neuromuscular disease will reduce the ventilatory drive and will lead to thereduction in both pAO2 and pAO2.