Which one is not a step of pharmacokinetics?

The correct answer is D: Synthesis. Pharmacokinetics involves the study of drug absorption, distribution, metabolism, and excretion (ADME). Synthesis is not a step of pharmacokinetics; it refers to the creation of new compounds rather than the processes of drug movement and transformation within the body. Absorption, distribution, and metabolism are integral components of pharmacokinetics as they determine a drug's concentration and effects in the body. In summary, the other choices (A, B, and C) are steps directly related to pharmacokinetics, while synthesis (D) is not part of this specific process.