**Core Concept**
The Minimum Alveolar Concentration (MAC) is a measure of the potency of an inhalational anesthetic agent, defined as the concentration of the agent in the lungs that is required to prevent movement in 50% of patients in response to a painful stimulus. A lower MAC value indicates a more potent anesthetic.

**Why the Correct Answer is Right**
Nitrous oxide (N2O) has a relatively low MAC value of 104%, making it the least potent inhalational anesthetic agent among the options. This is because N2O has a low solubility in blood and tissues, allowing it to reach the site of action in the brain more quickly and efficiently. This results in a rapid onset of anesthesia and a relatively low MAC value.

**Why Each Wrong Option is Incorrect**
**Option A:** Isoflurane has a MAC value of approximately 1.2%, making it one of the most potent inhalational anesthetics. Its high potency is due to its high solubility in blood and tissues, which slows down the onset of anesthesia.

**Option B:** Sevoflurane has a MAC value of approximately 2%, making it less potent than isoflurane but still more potent than nitrous oxide. Its high potency is due to its ability to bind to the gamma-aminobutyric acid (GABA) receptor, which is the primary site of action for most anesthetic agents.

**Option C:** Desflurane has a MAC value of approximately 6%, making it less potent than sevoflurane but still more potent than nitrous oxide. Its high potency is due to its ability to rapidly equilibrate between the blood and tissues, allowing it to reach the site of action in the brain quickly.

**Clinical Pearl / High-Yield Fact**
The MAC value of an anesthetic agent can be affected by various factors, including age, sex, and body temperature. For example, the MAC value of halothane decreases with increasing age, while the MAC value of isoflurane increases with decreasing body temperature.

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