**Core Concept**
Nitrous oxide (N2O) is a colourless, sweet-smelling inhalational anaesthetic agent used for its analgesic and anaesthetic properties. It is not metabolized by the body, making it a unique agent in the context of anaesthesia.

**Why the Correct Answer is Right**
Nitrous oxide is not metabolized by the body because it is not significantly affected by the cytochrome P450 enzyme system, which is responsible for the metabolism of most other anaesthetic agents. As a result, N2O is exhaled unchanged, with a small fraction being exhaled as nitric oxide and nitrogen gas. This characteristic makes N2O a valuable agent for its rapid onset and offset of action, as well as its minimal impact on the body's own metabolic pathways.

**Why Each Wrong Option is Incorrect**
**Option B:** Gallamine is a non-depolarizing muscle relaxant, not an anaesthetic agent. It is metabolized by the liver and excreted in the urine.
**Option C:** Sevoflurane is an inhalational anaesthetic agent, but it is metabolized by the liver to a small extent, producing inorganic fluoride ions.
**Option D:** Halothane is also an inhalational anaesthetic agent, but it undergoes significant hepatic metabolism, producing trifluoroacetic acid and other metabolites.

**Clinical Pearl / High-Yield Fact**
It's worth noting that N2O can displace oxygen in the body, leading to a condition known as diffusion hypoxia. This is particularly concerning in patients who have been breathing 100% N2O for an extended period, as it can lead to hypoxia and even cardiac arrest.

✓ Correct Answer: A. N2O