**Core Concept**
The therapeutic index is a pharmacological concept that measures the relative safety of a drug. It is defined as the ratio of the dose of a drug that produces toxicity to the dose that produces a clinically desired or effective response.

**Why the Correct Answer is Right**
The therapeutic index is a quantitative measure that compares the blood concentration at which a drug becomes toxic and the concentration at which the drug is effective. A higher therapeutic index indicates a safer drug with a larger margin between the effective dose and the toxic dose.

**Why Each Wrong Option is Incorrect**
**Option A:** This choice is incorrect because the therapeutic index is not a direct measure of drug efficacy.
**Option B:** This choice is incorrect because it does not accurately describe the therapeutic index.
**Option C:** This choice is incorrect because the therapeutic index is specifically related to the safety margin of a drug.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that drugs with a high therapeutic index are generally safer and have a lower risk of toxicity, while drugs with a low therapeutic index require careful dosing and monitoring.

**Correct Answer:** D. Therapeutic index is a measure of the relative safety of a drug.