**Core Concept**
The index of potency of general anaesthesia refers to a quantitative measure that compares the relative potency of different anaesthetic agents. It is a crucial concept in anaesthesiology, as it helps anaesthetists to select the most appropriate agent for a specific patient.

**Why the Correct Answer is Right**
The correct index of potency of general anaesthesia is the Minimum Alveolar Concentration (MAC). MAC is the concentration of an inhaled anaesthetic in the lungs that is required to prevent movement in 50% of patients in response to a surgical stimulus. It is a widely accepted measure of anaesthetic potency, with lower MAC values indicating higher potency. MAC is influenced by various factors, including the solubility of the anaesthetic in blood and tissues, its rate of onset and offset, and its mechanism of action.

**Why Each Wrong Option is Incorrect**
**Option A:** MAC is not the same as Minimum Alveolar Concentration for analgesia (MAC-A), which is a measure of an anaesthetic's analgesic potency.
**Option B:** The Potency Ratio (PR) is a related concept, but it is not the same as MAC. PR is the ratio of the MAC of two anaesthetics, and it is used to compare their relative potencies.
**Option C:** The Effective Dose 50 (ED50) is a measure of the dose of an anaesthetic required to produce a specific effect in 50% of patients, but it is not a direct measure of anaesthetic potency.

**Clinical Pearl / High-Yield Fact**
A classic exam trap is to confuse MAC with Minimum Alveolar Concentration for analgesia (MAC-A). Remember that MAC is a measure of anaesthetic potency, while MAC-A is a measure of analgesic potency.

**Correct Answer: D. MAC**