Which of the following best describes a ligand?

A ligand is best described as a molecule that binds to a receptor. Ligands play a crucial role in the field of pharmacology and biochemistry, as they initiate biological responses by attaching to specific receptors on cells. This binding can lead to various outcomes, such as activating or inhibiting signaling pathways that result in physiological changes.

In the context of pharmacotherapy, ligands can be endogenous substances, like hormones and neurotransmitters, or exogenous compounds, including drugs. The interaction between a ligand and its receptor is often very specific, influencing how drugs are designed to target particular conditions based on their mechanism of action.

The other options, while they pertain to different aspects of drug action and biological processes, do not accurately define a ligand. Hence, identifying a ligand as a molecule that binds to a receptor is fundamental to understanding drug-receptor interactions and their implications in therapeutic effects.