Which organ is primarily involved in excretion of drug metabolites?

• A. Stomach
• B. Heart
• C. Kidneys
• D. Lungs

Answer: (C)

Drug excretion of metabolites is dominated by the kidneys. After drugs are metabolized—often into more polar, water-soluble forms—the kidneys filter the blood and actively secrete these metabolites into the tubular fluid, which becomes urine. This renal excretion is aided by glomerular filtration and tubular transport processes, with some substances also undergoing reabsorption. While the liver can form metabolites and the biliary system may excrete some into feces, and the lungs can excrete certain volatile compounds, the kidneys are the primary route for eliminating most drug metabolites.