**Core Concept**
The index of potency of general anesthetics is a measure of the relative effectiveness of different agents in producing a desired level of anesthesia. This concept is crucial in understanding the pharmacodynamics of anesthetics and their clinical applications.

**Why the Correct Answer is Right**
The index of potency is typically expressed as the dose of anesthetic required to produce 50% of the maximum possible effect (ED50). For inhalational anesthetics, the index of potency is inversely related to the minimum alveolar concentration (MAC) required to prevent movement in response to a painful stimulus. Agents with a lower MAC are considered more potent. The correct answer, MAC, is a widely accepted measure of the potency of inhalational anesthetics.

**Why Each Wrong Option is Incorrect**
**Option A:** ED90 - While ED90 is a related concept, it represents the dose required to produce 90% of the maximum effect, not the index of potency. ED90 is not a measure of potency, but rather a measure of the dose required to achieve a specific level of effect.
**Option B:** Blood-gas partition coefficient - This coefficient is a measure of the solubility of an anesthetic in blood, which affects its induction and recovery characteristics. However, it is not a direct measure of potency.
**Option C:** Clearance - Clearance is a measure of the rate at which an anesthetic is removed from the body, but it does not directly relate to its potency.

**Clinical Pearl / High-Yield Fact**
Remember that the MAC of an inhalational anesthetic is influenced by factors such as age, temperature, and the presence of certain disease states. A lower MAC indicates greater potency, but it does not necessarily imply a safer or more desirable anesthetic agent.

**Correct Answer: B. Blood-gas partition coefficient is not the correct answer, however the correct answer to the question is MAC, however I am unable to provide the correct answer letter as it is missing.