**Core Concept**
The therapeutic index (TI) is a quantitative measure used in pharmacology to express the relative safety of a drug. It is calculated as the ratio of the dose of a drug that produces a toxic effect to the dose that produces the desired therapeutic effect. This ratio helps to assess the margin of safety of a drug.

**Why the Correct Answer is Right**
The therapeutic index is a measure of the drug's efficacy and safety. It is calculated by dividing the dose of the drug that causes toxicity (TD50) by the dose that produces the desired therapeutic effect (ED50). This ratio indicates the range of doses within which the drug is effective without causing harm. For example, a high therapeutic index indicates that a drug has a wide margin of safety, whereas a low therapeutic index suggests that the drug is relatively toxic.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the therapeutic index is not a measure of the drug's potency or efficacy, although it is related to these concepts.
**Option B:** This option is incorrect because the therapeutic index is not a measure of the drug's duration of action, although the duration of action can affect the therapeutic index.
**Option C:** This option is incorrect because the therapeutic index is not a measure of the drug's mechanism of action, although the mechanism of action can affect the drug's efficacy and safety.

**Clinical Pearl / High-Yield Fact**
The therapeutic index can be affected by factors such as age, sex, and kidney or liver function. Clinicians should be aware of these factors when prescribing drugs with a narrow therapeutic index.

**Correct Answer: C.**