Which of the following best exemplifies a genetic factor contributing to pharmacokinetic variability?

• A. Change in number/function of enzymes involved in drug metabolism
• B. Change in receptor density
• C. Patient's expectation of drug effect
• D. Temperature of storage

Answer: (B)

Genetic factors that drive pharmacokinetic variability are those that alter how the body processes a drug—absorption, distribution, metabolism, and elimination. The classic example is variation in the number or function of drug-metabolizing enzymes. Differences in enzymes like the cytochrome P450 family can speed up or slow down metabolism, changing plasma concentrations, half-life, and overall drug exposure. This directly affects clearance and pharmacokinetics. In contrast, changes in receptor density affect pharmacodynamics—how the body responds to a given concentration—while a patient’s expectation (placebo effect) or storage temperature influence pharmacodynamics or handling, not genetic disposition to drug disposition. So, variation in metabolic enzyme activity best exemplifies a genetic factor impacting pharmacokinetic variability.