Explain how a ligand can be a competitive antagonist at one receptor and an agonist at another.

• A. Ligands can have different receptor conformations and signaling environments across receptor types, leading to divergent actions.
• B. A ligand cannot have different actions at different receptors.
• C. All receptors respond identically to every ligand.
• D. Competitive antagonists always block all receptors.

Answer: (A)