Pharmacodynamics is best defined as:

Pharmacodynamics refers to the study of how a drug affects an organism, particularly concerning the relationship between drug concentration and its therapeutic or toxic effects. The correct answer focuses on the interaction between the drug and non-target sites, which plays a critical role in understanding side effects and the overall efficacy of the drug.

This option embodies a broad understanding of drug action, encompassing not just the desired effects at target sites but also how a drug may unintentionally interact with other sites in the body, leading to potential side effects or altered therapeutic outcomes.

In pharmacodynamics, these interactions are crucial for predicting the overall pharmacological response. Understanding these dynamics helps clinicians make informed decisions about dosage, administration routes, and potential adverse effects. Thus, this definition encapsulates the broader aspects of how drugs work within the body beyond the primary targets, enabling a more comprehensive understanding of their pharmacologic action.