The ratio of the median lethal dose of the median effective dose is the
**Core Concept**
The therapeutic index (TI) is a pharmacological measure used to compare the efficacy and safety of a drug by expressing the ratio of the median lethal dose (LD50) to the median effective dose (ED50). It helps in assessing the risk-benefit profile of a drug and in determining its potential for causing adverse effects.
**Why the Correct Answer is Right**
The therapeutic index is calculated as the ratio of the LD50 to the ED50. The LD50 is the dose of a drug that is lethal to 50% of the test subjects, while the ED50 is the dose that produces the desired therapeutic effect in 50% of the test subjects. By comparing these two values, the TI provides a quantitative measure of a drug's safety and efficacy. For example, a high TI indicates that a drug is relatively safe, while a low TI suggests that it has a narrow therapeutic index and may be more toxic.
**Why Each Wrong Option is Incorrect**
**Option A:** The morbidity index is not a pharmacological measure and is not related to the ratio of LD50 to ED50. Morbidity refers to the rate of disease or illness in a population.
**Option B:** The mortality index is a measure of the number of deaths in a population, but it is not related to the ratio of LD50 to ED50. It is not a pharmacological measure.
**Option C:** The anesthetic ratio is not a standard pharmacological measure and is not related to the ratio of LD50 to ED50. Anesthesia is a specific area of pharmacology, but the anesthetic ratio is not a widely recognized term.
**Clinical Pearl / High-Yield Fact**
When selecting a medication, remember that a high therapeutic index (TI) indicates a relatively safe drug with a wide margin of safety, while a low TI suggests a narrow margin of safety and a greater risk of toxicity.
**✓ Correct Answer: D. Therapeutic index**