Which condition results in a decreased PAO2-PaO2 difference?

The PAO2-PaO2 difference refers to the difference between the partial pressure of oxygen in the alveoli (PAO2) and the partial pressure of oxygen in the arterial blood (PaO2). A decrease in this difference suggests that there is a reduced capacity for oxygen transfer from the alveoli to the blood.

Ventilation defects can lead to a decreased PAO2-PaO2 difference because they imply that there is inadequate ventilation in certain areas of the lungs. This can result in lower oxygen levels in the alveoli, which subsequently leads to lower arterial oxygen levels. When ventilation is compromised, it diminishes the oxygen available for transfer to the blood, thus decreasing the difference between PAO2 and PaO2.

On the other hand, arteriovenous shunting (A/V shunting) typically leads to increased PaO2 as blood bypasses the ventilated areas of the lung, while V/Q inequality can cause varying effects based on the degree of mismatch but usually does not solely result in a decreased PAO2-PaO2 difference. Therefore, the best condition for describing a decrease in this difference, as indicated in the question, is due to ventilation defects.