Which apical extension of epithelial cells in the kidney increases surface area and contains enzymes and transporters?

• A. Cilia
• B. Stereocilia
• C. Flagella
• D. Microvilli (brush border)

Answer: (D)

The key idea is that the apical surface of certain kidney cells is specialized to maximize reabsorption. Microvilli form a dense, brush-border extension on these cells, dramatically increasing surface area so more filtrate can be reabsorbed. This enlarged surface also hosts membrane-bound enzymes and transporters that process substances and move them from the filtrate back into the bloodstream. In the proximal tubule of the kidney, this brush border is essential for reclaiming water, glucose, amino acids, and ions efficiently.

Other apical extensions serve different roles. Cilia are slender, beat to move luminal contents or sense flow rather than mainly increasing surface area. Stereocilia are long microvilli found in tissues like the epididymis and inner ear, not typically in kidney tubules. Flagella are motile tails used for cell movement, not for absorption. So the microvilli forming the brush border best fit the description of increasing surface area and containing enzymes and transporters in kidney epithelial cells.