**Core Concept**
The second gas effect is a phenomenon where the partial pressure of an inhalational anesthetic agent increases due to the displacement of nitrogen from alveoli by the first gas administered, thereby enhancing the anesthetic effect of the first gas.

**Why the Correct Answer is Right**
The second gas effect is particularly notable when a potent inhalational anesthetic is administered after nitrous oxide, as the latter has a high blood-gas partition coefficient. When nitrous oxide is inhaled, it rapidly increases the partial pressure of anesthetic gas in the alveoli, which then facilitates the uptake of the subsequent anesthetic agent. This effect is attributed to the displacement of nitrogen from the alveoli by nitrous oxide, resulting in an increase in the partial pressure of the second anesthetic agent.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect as it does not describe a scenario where the second gas effect would occur. The second gas effect requires the administration of a potent inhalational anesthetic after a gas like nitrous oxide that has a high blood-gas partition coefficient.
* **Option B:** This option is incorrect as it does not accurately describe the mechanism behind the second gas effect. The second gas effect is not related to the blood-brain barrier, but rather to the displacement of nitrogen from the alveoli by the first gas administered.
* **Option C:** This option is incorrect as it does not provide a specific gas that would be involved in the second gas effect. The second gas effect typically involves a potent inhalational anesthetic agent, such as halothane or isoflurane, which is administered after a gas like nitrous oxide.

**Clinical Pearl / High-Yield Fact**
When administering inhalational anesthetics, it is essential to consider the second gas effect to avoid excessive anesthetic concentrations and potential complications.

**Correct Answer: D. Nitrous oxide.**