Which is an example of pharmacodynamics in action?

Pharmacodynamics refers to the study of how a drug affects the body, encompassing the mechanisms of action, the relationship between drug concentrations and effects, and the resultant physiological changes. Among the examples provided, a drug causing an increase in heart rate directly illustrates pharmacodynamics in action, as it showcases the effect of the drug on a specific bodily function.

While the other examples, like the body metabolizing a drug, are related to pharmacokinetics, which focuses on how the body processes the drug, and side effects are also outcomes of drug actions, the most direct reflection of pharmacodynamics is in the drug's intended effect, such as increased heart rate.

Since an increase in heart rate as a result of drug administration directly demonstrates the drug's action on the body, it is the most representative example of pharmacodynamics. Therefore, the choice of all examples collectively implies that they are either directly or indirectly related to how drugs interact with the body, affirming a broader understanding of the drug effects involved in this discipline.