**Core Concept**
In asthma, hypoxemia occurs due to a specific pathophysiological mechanism involving the mismatch between alveolar ventilation and perfusion. This is a critical concept in understanding the respiratory consequences of asthma exacerbations.

**Why the Correct Answer is Right**
During an asthma attack, airway inflammation and constriction lead to airway obstruction, which impairs gas exchange. As a result, alveoli receive less air (ventilation) than they would under normal conditions, while perfusion remains relatively constant. This mismatch between ventilation and perfusion leads to a decrease in oxygen levels in the blood (hypoxemia). The alveolar-capillary membrane remains intact, and diffusion is not significantly affected in asthma. Therefore, the primary mechanism of hypoxemia in asthma is a ventilation-perfusion mismatch.

**Why Each Wrong Option is Incorrect**
**Option A:** Hypoventilation is a contributing factor to hypoxemia in asthma but is not the primary mechanism. While hypoventilation does occur due to airway obstruction, it is not the sole reason for the hypoxemia.

**Option B:** Diffusion is not significantly decreased in asthma. The alveolar-capillary membrane remains intact, and oxygen diffusion across the membrane is not impaired. Therefore, diffusion is not a primary mechanism of hypoxemia in asthma.

**Option C:** Shunting is not a primary mechanism of hypoxemia in asthma. Shunting typically occurs in conditions with significant pulmonary vascular disease or atelectasis, where blood passes through alveoli that are not participating in gas exchange. In asthma, the primary issue is a ventilation-perfusion mismatch, not shunting.

**Clinical Pearl / High-Yield Fact**
A key clinical correlation to remember is that asthma exacerbations can lead to significant hypoxemia, even in the absence of overt respiratory failure. This is because the ventilation-perfusion mismatch can occur early in the course of an asthma attack, leading to decreased oxygen levels in the blood.

**✓ Correct Answer: D. Ventilation perfusin mismatch**