## Core Concept
The potency of an inhalational anaesthetic agent is a measure of its ability to induce anaesthesia at a specific concentration. It is closely related to the **lipid solubility** of the agent, which reflects its ability to interact with the lipid bilayer of neuronal membranes. This concept is fundamental in understanding how inhalational anaesthetics work.

## Why the Correct Answer is Right
The correct answer, **minimum alveolar concentration (MAC)**, is a direct measure of anaesthetic potency. MAC is defined as the concentration of vapour in the lungs that is needed to prevent movement in 50% of subjects in response to a painful stimulus, such as a surgical incision. A lower MAC value indicates higher potency because less anaesthetic is required to achieve the desired effect. The relationship between MAC and potency is rooted in the fact that agents with lower MAC values have higher lipid solubility, allowing them to more easily interact with and disrupt neuronal membranes.

## Why Each Wrong Option is Incorrect
- **Option A:** While related to anaesthetic pharmacokinetics, **blood/gas partition coefficient** primarily affects the **speed of induction and recovery** rather than the potency of the anaesthetic. It reflects how the anaesthetic is distributed between blood and alveolar air but does not directly measure potency.
- **Option B:** **Oil/water partition coefficient** is a measure that correlates with potency, as it reflects the lipid solubility of the anaesthetic. However, it is not the direct measure of potency as defined by clinical practice.
- **Option D:** **Blood/brain partition coefficient** influences the rate of induction of anaesthesia and recovery but is not a direct measure of anaesthetic potency.

## Clinical Pearl / High-Yield Fact
A key point to remember is that **the lower the MAC of an inhalational anaesthetic, the higher its potency**. For example, desflurane has a high MAC (around 6%) indicating lower potency compared to sevoflurane or isoflurane, which have lower MAC values (around 2% and 1.15%, respectively). This relationship helps in choosing the appropriate anaesthetic agent based on the patient's needs and the type of surgery.

## Correct Answer: C. Minimum alveolar concentration (MAC).