Final answer:
The brush border consists of the dense microvilli on the apical membrane of the proximal tubule epithelial cells in the kidney, which increase surface area for absorption. The correct answer to the question is '4)
Step-by-step explanation:
The brush border refers to the densely packed microvilli on the apical membrane of certain epithelial cells in the kidney, specifically those lining the Proximal Convoluted Tubule (PCT).
These microvilli significantly increase the surface area for absorption, allowing for more efficient reabsorption of solutes like sodium, chloride, and glucose, which is vital for the kidney's function.
Therefore, the correct answer to the question is '4) microvilli on the apical membrane of the proximal tubule'. It plays an essential role in the kidney's ability to reabsorb needed substances and substances back into the bloodstream.