Final answer:
Microvilli are described collectively as a brush border because their combined microscopic appearance resembles a mass of bristles. The brush border greatly increases the surface area of the small intestine, enhancing nutrient absorption.
Step-by-step explanation:
Microvilli are small cylindrical extensions of the plasma membrane of the mucosa's epithelial cells in the small intestine. They are collectively described as a brush border due to their microscopic appearance, which resembles a mass of bristles. The brush border is created by the combined appearance of many microvilli. It greatly increases the surface area of the small intestine, enhancing the absorption of nutrients.