Which statement correctly describes pharmacodynamics?

A. How drugs move throughout the body: The movement of drugs through the body refers to pharmacokinetics, which includes absorption, distribution, metabolism, and excretion. This field examines how the body handles a drug after administration. It focuses on plasma concentrations, bioavailability, and elimination rather than receptor-level effects. B. How the drug gets into the bloodstream: Entry of a drug into the bloodstream is specifically the process of absorption, which is one component of pharmacokinetics. Absorption depends on factors such as route of administration, membrane permeability, blood flow, and drug solubility. It does not describe the biologic response produced after the drug reaches systemic circulation. C. The process of changing active drug into inactive metabolites: Conversion of active drug into metabolites occurs primarily in the liver through enzymatic pathways such as the cytochrome P450 system. This process is called metabolism, another component of pharmacokinetics. It explains how drugs are chemically altered but not how they exert their effects at target sites. D. What the drug does to the body to cause an effect: Pharmacodynamics describes the biochemical and physiologic effects of drugs and their mechanisms of action at receptor sites. It includes receptor binding, signal transduction, dose-response relationships, and therapeutic versus adverse effects. This field explains how drug–receptor interactions translate into measurable clinical outcomes.