Which statement describes the primary route of excretion for medications and their metabolites?

• A. Excretion via the lungs
• B. Biliary excretion into bile only
• C. Primarily through the kidneys via glomerular filtration, active tubular secretion, and passive tubular reabsorption
• D. Excretion through sweat glands only

Answer: (C)

Renal excretion through the kidneys is the primary route for most medications and their metabolites. The kidneys clear drugs via three coordinated processes: glomerular filtration, which passively filters unbound drug into the filtrate; active tubular secretion, where transporters in the proximal tubule actively move certain drugs into the urine to boost elimination; and passive tubular reabsorption, where lipid-soluble, nonionized forms of a drug can cross back into the blood, reducing excretion. The balance of these processes, along with the drug’s degree of protein binding, lipophilicity, and ionization (which is influenced by urine pH), largely determines how quickly a drug is cleared. While other routes exist—biliary excretion into bile for some drugs, exhalation via the lungs for volatile substances, or minor routes like sweat—the kidneys handle the majority of drug elimination for most medications.