**Core Concept**
The second gas effect is a phenomenon observed during anesthesia, where the uptake of a second gas is accelerated by the presence of a first gas, typically nitrous oxide (N2O). This occurs due to the rapid elimination of N2O from the alveoli, creating a concentration gradient that facilitates the uptake of other gases.

**Why the Correct Answer is Right**
Nitrous oxide has a high blood-gas partition coefficient, which allows it to rapidly diffuse out of the blood and into the alveoli. As a result, the alveolar concentration of N2O decreases, creating a concentration gradient that facilitates the uptake of other gases, such as anesthetics. This phenomenon is known as the second gas effect. The rapid elimination of N2O from the blood also leads to a decrease in cerebral blood flow, which can exacerbate the effects of other anesthetics.

**Why Each Wrong Option is Incorrect**
**Option A:** Ether, an older anesthetic, is not typically associated with the second gas effect, as it has a slower onset and offset of action compared to modern anesthetics.
**Option C:** Desflurane, a halogenated anesthetic, has a low blood-gas partition coefficient, which makes it less likely to exhibit the second gas effect.
**Option D:** Sevoflurane, another halogenated anesthetic, also has a low blood-gas partition coefficient, similar to desflurane, and is not typically associated with the second gas effect.

**Clinical Pearl / High-Yield Fact**
The second gas effect is an important consideration when using nitrous oxide in combination with other anesthetics, as it can lead to rapid changes in anesthetic concentrations and potentially adverse effects.

**✓ Correct Answer: B. Nitrous oxide**