**Core Concept**
Diffusion hypoxia is a condition that occurs when a patient breathes in a mixture of gases that has been enriched with oxygen during anesthesia, leading to a rapid increase in oxygen partial pressure in the lungs. This causes oxygen to diffuse out of the lungs into the bloodstream, reducing the partial pressure of oxygen in the alveoli and impairing gas exchange.

**Why the Correct Answer is Right**
The correct answer is related to the phenomenon of diffusion hypoxia occurring during the recovery phase from anesthesia, specifically when the patient is breathing 100% oxygen. This is because the high concentration of oxygen in the lungs causes a rapid increase in oxygen diffusion into the bloodstream, leading to a decrease in alveolar oxygen partial pressure. The mechanism involves the diffusion of oxygen from the alveoli into the blood, which can lead to a significant decrease in the partial pressure of oxygen in the alveoli, impairing gas exchange.

**Why Each Wrong Option is Incorrect**
* **Option A:** During induction of anesthesia, the patient is not breathing 100% oxygen and is not at risk for diffusion hypoxia.
* **Option B:** During mechanical ventilation, the patient is not breathing spontaneously and is not at risk for diffusion hypoxia.
* **Option C:** During exercise, the patient is not receiving supplemental oxygen and is not at risk for diffusion hypoxia.

**Clinical Pearl / High-Yield Fact**
To avoid diffusion hypoxia, it is essential to gradually wean patients from 100% oxygen during recovery from anesthesia, allowing the partial pressure of oxygen in the alveoli to gradually increase to normal levels.

**Correct Answer: D. During recovery from anesthesia when breathing 100% oxygen.**