Which option best completes the mnemonic SAD to describe the three main functions of simple epithelial cells?

• A. Secretion (cuboidal)
• B. Absorption (columnar)
• C. Diffusion (squamous)
• D. Secretion, Absorption, and Diffusion

Answer: (D)

The main idea being tested is that simple epithelial cells perform three broad tasks: secretion, absorption, and diffusion. The mnemonic SAD helps you remember these three functions across the different simple epithelial types. Secretion is common in glandular or ductal epithelia, often lined by cuboidal cells; absorption is a hallmark of absorptive epithelia like the intestinal lining, typically columnar; diffusion happens across very thin barriers formed by simple squamous epithelium, such as in the alveoli or capillaries. Because the same tissue family includes cells specialized for each of these roles, the complete phrase Secretion, Absorption, and Diffusion best describes the trio of main functions rather than tying a single function to one cell shape. The other options try to pin one function to a specific shape, which doesn’t capture the full range of activities of simple epithelium.