What does pharmacodynamic variability refer to?

• A. Differing concentrations at the site of action
• B. Variations in absorption rate
• C. Differences in metabolic rate
• D. Differing response to the same concentration at the site of action

Answer: (D)

Pharmacodynamics looks at what the drug does to the body, including how binding at the site of action translates into a response. Variability in pharmacodynamics means that the same drug concentration at the site of action can produce different effects in different individuals. This happens when there are differences in receptor sensitivity or density, variations in post-receptor signaling pathways, or other factors that alter how the body responds to the drug, independent of how much drug actually reaches the site.

So the statement that there can be a differing response to the same concentration at the site of action is the best description. Differences in absorption rate or metabolic rate change how much drug gets to the site (pharmacokinetic factors), not the response to a given concentration.