Which is the largest organ by surface area?

• A. Liver
• B. Skin
• C. Brain
• D. Heart

Answer: (B)

The key idea is surface area: which organ has the most area exposed on the body's exterior. The skin covers the entire outside of the body, giving it a large surface area — about 1.5 to 2 square meters in an average adult. This expansive area supports its roles as a protective barrier, a regulator of body temperature through sweating and blood flow, and a sensitive sense organ. Internal organs like the liver can be very large in mass, but their surfaces aren’t exposed externally to the same extent, so they don’t contribute as much to surface area. The brain’s surface is highly folded to increase area, but the total surface is still far smaller than the skin’s. The heart’s surface is even smaller. Therefore, skin is the largest organ by surface area.