______ is a quantitative assessment of the safety of a specific drug.

• A. Toxic dose
• B. Bioavailability
• C. Therapeutic index
• D. Clearance

Answer: (C)

The concept here is the therapeutic index, a quantitative measure of a drug’s safety margin. It’s defined as the ratio of a dose that produces toxicity (often TD50) to a dose that produces a therapeutic effect (ED50). This number tells you, on average, how much you can increase the dose before toxicity becomes likely compared to the dose needed for therapeutic benefit. A high therapeutic index means a wide safety margin—the drug can be used more safely with less risk of accidental toxicity. A low therapeutic index means the safety margin is narrow, so small changes in dose or patient factors can push toward toxicity, requiring careful dosing and monitoring.

Other terms don’t capture this safety margin in the same way. Toxic dose describes a dose that causes harm but doesn’t quantify how far it is from the effective dose. Bioavailability concerns how much of the drug reaches systemic circulation, and clearance relates to how quickly the drug is removed from the body. They’re important pharmacokinetic concepts, but they don’t provide a single numeric safety ratio like the therapeutic index.