If a drug is eliminated via significant renal tubular secretion through transporter X, and another drug inhibits transporter X, what is the expected effect on renal clearance?

• A. CLrenal decreases
• B. CLrenal increases
• C. CLrenal unchanged
• D. Only filtration changes

Answer: (A)

The key idea is that renal clearance includes active tubular secretion in addition to filtration. If a drug is cleared largely by secretion through a transporter, that secretory path adds a significant amount to how much drug is eliminated by the kidney. When another drug inhibits that transporter, the secretory component is reduced or eliminated, so less drug is cleared per unit time. Filtration itself (the passive process) remains the same, assuming glomerular filtration rate and protein binding don’t change, but the overall amount cleared via the kidney decreases because the secretory pathway is blocked. So renal clearance decreases.

The other options don’t fit because inhibiting a transporter involved in secretion does not increase clearance, and it does change clearance since secretion contributed notably to elimination. It’s not correct to say only filtration changes because the inhibition specifically affects the secretory process, not the filtration rate.