Which of the following is the largest organ of the human body by surface area, the one often cited in anatomy texts?

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

Answer: (D)

The thing being tested is which organ has the largest surface area. The skin fits that description because it forms the body's outer boundary and covers essentially the entire external surface. In an average adult, the skin’s area is about 1.5 to 2 square meters (and can be larger in bigger individuals), making it larger in surface area than any other single organ.

Other organs are large in their own right, but their surfaces are mostly internal. The heart is a compact pump with a relatively small external area compared with the skin. The liver is large in mass, but its surface is internal to the body’s cavities. The lungs do have a vast internal surface area for gas exchange—often cited as substantial—but that surface belongs to internal organs and is not the body’s external surface.

So, the skin is the largest organ by surface area.