Where are stereocilia located and what is their role in the epididymis?

• A. In the trachea they move mucus upward
• B. In the kidney they function as primary cilia
• C. In the epididymis they are tall microvilli that expand absorptive surface area for fluid reabsorption
• D. In the intestine they increase surface area for absorption

Answer: (C)

Stereocilia in the epididymis are long, non-motile apical projections of epithelial cells that dramatically increase the surface area available for reabsorbing fluid from the lumen. This fluid reabsorption concentrates and helps mature the sperm as they pass through the epididymis. The epididymal epithelium relies on these tall microvilli-like extensions to reclaim most of the luminal fluid, which is essential for proper sperm maturation.

In contrast, mucus-moving cilia in the trachea are motile and handle mucus clearance, not fluid reabsorption in the epididymis. Primary cilia in the kidney refer to sensory organelles, not stereocilia. And in the intestine, surface area is increased by microvilli for absorption, but those are microvilli, not stereocilia, and the question specifically about the epididymis.