Kd?','choices':['Drug association','Drug dissociation constant','Rate constant for receptor internalization','Drug dissociation'],

• A. Drug association
• B. Drug dissociation constant
• C. Rate constant for receptor internalization
• D. Drug dissociation

Answer: (D)

Kd is the equilibrium dissociation constant for a drug–receptor interaction. It quantifies how tightly the drug remains bound by describing the balance between binding and unbinding at equilibrium. Mathematically, it equals koff/kon and, at equilibrium, [D][R]/[DR]. The smaller the Kd, the higher the affinity, meaning the drug stays bound at lower concentrations. Therefore, the phrase that best matches Kd is the drug dissociation constant. The other terms describe the process or a different concept (dissociation as a process, association as binding, or a rate constant for internalization) rather than the constant itself.