Which of the following is a pharmacokinetic mechanism contributing to tolerance?

• A. Downregulation of receptors
• B. Increased clearance rate
• C. Pharmacodynamic adaptations
• D. Decreased concentration of drug at target

Answer: (B)

Tolerance can arise from changes in how the body handles a drug (pharmacokinetics) or from changes at the drug’s target (pharmacodynamics). The correct mechanism is increased clearance rate, because speeding up the elimination of the drug reduces its overall exposure and the amount reaching the target after repeated dosing, leading to a diminished effect. This reflects a change in ADME (such as enzyme induction that boosts metabolism or enhanced renal excretion) that lowers drug concentrations at the site of action over time. Downregulation of receptors is a pharmacodynamic adaptation, altering response at the target rather than how the body processes the drug. Decreased concentration at the target is an outcome of pharmacokinetic or distribution changes, not by itself the mechanism generating tolerance.