What best defines a drug's selectivity?

• A. A measure of the drug's affinity for all receptors
• B. The maximum effect the drug can produce
• C. The rate at which the drug is eliminated
• D. A measure of the drug's ability to solely interact with its intended target

Answer: (D)

Selectivity is about a drug preferring its intended target over other possible targets, so it acts mainly where it’s meant to and causes fewer off-target effects. The best phrasing captures that idea by describing the drug’s ability to solely interact with its intended target, which is what we mean by high selectivity. If a drug bound many receptors with similar affinity, its selectivity would be low, and off-target effects would be more likely. This is different from affinity (how tightly it binds to receptors), efficacy (the maximum effect once bound), or pharmacokinetics like how fast it’s eliminated. So a drug’s selectivity focuses on target preference, not just how strong the binding is or how long the drug stays in the body.