Ka?','choices':['Drug dissociation constant','Equilibrium constant for receptor expression','Drug association','Rate constant for receptor internalization'],

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

Answer: (C)

Ka is the association constant for binding of a drug to its receptor—the equilibrium constant for the formation of the drug–receptor complex from free drug and free receptor. At equilibrium, Ka = [DR] / ([D][R]). This makes the correct choice the drug association, since Ka directly describes how readily the drug and receptor come together to form the bound complex. It is the inverse of the dissociation constant, Kd, which is [D][R]/[DR]; a larger Ka means tighter binding (higher affinity). Ka is related to the rate constants by Ka = kon/koff, with Kd = koff/kon. It is not an expression-related equilibrium constant or a rate constant for internalization.