Define pharmacodynamics.

Pharmacodynamics is primarily concerned with the effects of drugs on the body and the mechanisms by which these effects occur. This involves understanding how a drug exerts its therapeutic effects, how it interacts with receptors, enzymes, or other cellular components, and the resulting biochemical and physiological changes in the body. It encompasses the relationship between drug concentration at the site of action and the magnitude of the effect, which can help in determining proper dosing regimens and predicting responses in different patient populations.

The other options focus on different aspects of pharmacology. One looks at drug absorption, which is defined under pharmacokinetics, specifically relating to how the body takes up drugs. Another option refers to analyzing drug interactions and side effects, which may be valuable but does not encompass the broader question of how drugs function within the body. The last choice pertains to measuring plasma drug levels, a pharmacokinetic concept that deals with the movement of drugs through the body rather than their mechanisms of action or effects.