Pharmacodynamic drug-drug interactions occur when one drug changes the response of target or non-target tissues to another drug. Which option best describes this type of interaction?

• A. The response of target or non-target tissues to another drug.
• B. The concentration of the other drug in the plasma.
• C. The rate of renal clearance of the other drug.
• D. The absorption of the other drug in the gut is unchanged.

Answer: (A)

Pharmacodynamic interactions are about how a drug changes the effect that another drug has on tissues, at the level of receptors and downstream responses. The best option captures this idea by focusing on the change in the tissue response itself, not on drug concentrations or disposition. In other words, one drug alters how strongly or weakly the other drug produces its effect, whether at the target site or somewhere else in the body.

The other ideas describe pharmacokinetic factors—how the body handles the drug. Plasma concentration, renal clearance, and absorption unchanged all relate to drug levels and how quickly or where the drug is absorbed, distributed, metabolized, and excreted, rather than to how tissues respond to the drug. Therefore, the option that centers on the tissue response to another drug best fits pharmacodynamic interactions.