Renal excretion of medications involves which processes?

• A. Glomerular filtration, active tubular secretion, and passive tubular reabsorption
• B. Biliary secretion into bile
• C. Hepatic metabolism
• D. Diffusion into the intestinal lumen

Answer: (A)

Renal excretion relies on three main processes within the nephron: glomerular filtration, active tubular secretion, and passive tubular reabsorption. In glomerular filtration, plasma filters into the nephron, and only the unbound portion of a drug passes into the filtrate. Active tubular secretion uses carrier proteins in the proximal tubule to actively move certain drugs from the blood into the filtrate, increasing their elimination. After filtration and potential secretion, drugs can be reabsorbed passively from the tubular lumen back into the bloodstream, especially if they are lipid-soluble and non-ionized; this reduces net excretion. The overall renal clearance of a drug reflects how these processes work together and can be influenced by factors like pH, drug properties, and protein binding.

Biliary secretion into bile is a hepatic process and leads to fecal excretion, not renal excretion. Hepatic metabolism modifies drugs to aid elimination but is not a renal excretion mechanism by itself. Diffusion into the intestinal lumen pertains to gut processes rather than kidney-based excretion.