**Core Concept**
The index of potency of general anesthesia refers to the measurement of the relative effectiveness of different anesthetic agents. It is an important concept in anesthesiology, as it helps to determine the optimal dosage and administration of anesthetic drugs.

**Why the Correct Answer is Right**
The index of potency is calculated using the ED50 (effective dose 50) values of anesthetic agents, which represent the dose required to produce a specified effect in 50% of the population. The ED50 values are then compared to determine the relative potency of each anesthetic agent. For example, if the ED50 value of agent A is 1 mg/kg and the ED50 value of agent B is 0.5 mg/kg, agent B is considered more potent than agent A.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the index of potency is not directly related to the duration of action of anesthetic agents. While duration of action may be an important consideration in anesthetic practice, it is not the primary factor in determining the index of potency.
**Option B:** This option is incorrect because the minimum alveolar concentration (MAC) is a measure of anesthetic potency, but it is not the same as the index of potency. MAC is a measure of the concentration of anesthetic gas required to prevent movement in 50% of patients in response to a surgical stimulus.
**Option C:** This option is incorrect because the index of potency is not a measure of the speed of onset of anesthetic action. While speed of onset may be an important consideration in anesthetic practice, it is not the primary factor in determining the index of potency.

**Clinical Pearl / High-Yield Fact**
It's worth noting that the index of potency is not a fixed value and can vary depending on the specific anesthetic agent and the population being studied. Additionally, the index of potency is not a direct measure of anesthetic safety, and other factors such as side effects and toxicity must also be considered when selecting an anesthetic agent.

**Correct Answer: C. MAC (Minimum Alveolar Concentration) is a measure of anesthetic potency.**