**Core Concept**
Diffusion hypoxia refers to a phenomenon where the rapid diffusion of gases, particularly nitrous oxide, into the lungs can dilute the oxygen concentration, leading to a decrease in oxygen saturation. This occurs when a patient is switched from a nitrous oxide-containing anesthetic to a pure oxygen or air mixture.

**Why the Correct Answer is Right**
The rapid diffusion of nitrous oxide into the lungs, combined with the rapid excretion of this gas from the bloodstream, can lead to a dilution effect on the oxygen concentration in the lungs. This is because nitrous oxide is more soluble in blood than oxygen. As a result, the partial pressure of oxygen (pO2) in the alveoli decreases, leading to a decrease in oxygen saturation.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because diffusion hypoxia is not typically associated with other anesthetic gases like halothane or sevoflurane.
**Option B:** This option is incorrect because hypoxia due to inadequate ventilation or respiratory depression is a different clinical scenario from diffusion hypoxia.
**Option C:** This option is incorrect because histotoxic hypoxia, also known as hypoxia due to tissue poisoning, is a different type of hypoxia that occurs when certain toxic substances, such as cyanide, inhibit the electron transport chain.

**Clinical Pearl / High-Yield Fact**
To avoid diffusion hypoxia, it is essential to gradually decrease the concentration of nitrous oxide in the anesthetic mixture before switching to a pure oxygen or air mixture.

**Correct Answer: D. Nitrous oxide**