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The therapeutic index is defined as the difference between the therapeutic and toxic concentrations of a drug. This concept is crucial in pharmacology as it provides insight into the safety margin of a medication. A higher therapeutic index indicates a broader margin between effective and harmful doses, suggesting that a drug is safer for use. Understanding the therapeutic index helps healthcare providers make informed decisions regarding the dosing of medications, balancing efficacy with the risk of potential toxicity.

While the range of effective dosages for medications highlights dosing considerations, it does not directly denote the relationship between therapeutic and toxic concentrations. The measure of drug safety is a broader term that could incorporate various factors, including therapeutic index but is not limited to it. The ratio of drug clearance rates pertains to the pharmacokinetics of a drug and does not define the therapeutic index. Thus, option A accurately reflects the concept of the therapeutic index.