Which organ is the primary site of drug metabolism according to the source material?

• A. Kidney
• B. Liver
• C. Lungs
• D. Spleen

Answer: (B)

Drug metabolism happens mostly in the liver because it houses a high concentration of drug‑metabolizing enzymes in hepatocytes, especially the cytochrome P450 system. These enzymes carry out Phase I reactions (like oxidation, reduction, and hydrolysis) that introduce or expose functional groups, followed by Phase II reactions (such as conjugation with glucuronide, sulfate, or glutathione) that make drugs more water‑soluble. This transformation prepares drugs for easier excretion, typically by the kidneys. The liver’s central role is reinforced by its position receiving drugs absorbed from the gut via the portal circulation and by its rich blood flow, which together enable rapid metabolism and, for orally taken drugs, a notable first‑pass effect that can reduce the amount reaching systemic circulation. While other organs can contribute to metabolism in specific cases (for example, some volatile compounds can be metabolized in the lungs), the liver remains the primary site for most drug metabolism.