Which apical modification is primarily associated with detection of stimuli?

• A. Cilia
• B. Keratinization
• C. Microvilli
• D. Stereocilia

Answer: (D)

The key idea is that apical surface projections differ in function, with stereocilia specialized for sensing mechanical stimuli. On sensory hair cells in the inner ear, these finger-like extensions are rich in actin and bend in response to fluid movement or sound waves. That bending opens mechanosensitive ion channels, converting mechanical energy into electrical signals that the nervous system can interpret as sound or head motion. That sensory, detector role is what makes stereocilia the correct choice.

In contrast, microvilli mainly expand surface area to boost absorption and secretion (as in the gut), cilia are involved in moving fluids or sensing signals in many epithelia (and can be motile or immotile depending on type), and keratinization strengthens and protects the surface rather than detecting stimuli.