Registered Practical Nurse (RPN) Practice Test

How is the 'affinity' of a drug measured?

1. By its duration of action

2. By its strength of interaction with receptors

3. By its rate of metabolism

4. By its therapeutic index

The correct answer is: By its strength of interaction with receptors

The affinity of a drug refers to the strength of the binding interaction between the drug and its target receptors. A higher affinity indicates that the drug binds more tightly and effectively to its receptor, which enhances its potency and effectiveness at lower concentrations. This measurement is crucial in pharmacology because it helps in understanding how drugs work at a molecular level and informs decisions about dosage and therapeutic applications. In contrast, the other options provided address different concepts related to drug action and safety. Duration of action pertains to how long a drug remains effective in the body, which is influenced by several factors, including metabolism and excretion, but it does not directly measure affinity. The rate of metabolism refers to how quickly the body processes the drug, impacting its overall effectiveness and duration but again not its affinity. The therapeutic index is a measure of drug safety, comparing the effective dose to the toxic dose, and is not a measure of how tightly a drug binds to its receptor. Therefore, focusing on the strength of interaction with receptors is the most accurate way to gauge drug affinity.