What are the tiny infoldings of the plasma membrane that create a brush border called?

The tiny infoldings of the plasma membrane that create a brush border are known as microvilli. Microvilli are extensions of the plasma membrane that increase the surface area of a cell, particularly in epithelial tissues, which is crucial for functions like absorption and secretion. This is especially significant in the intestines, where increased surface area enhances nutrient absorption efficiency.

Microvilli consist of a core of actin filaments, which provide structural support and help maintain their shape. Their main role is to maximize the interaction of the cell with its environment, thereby facilitating the uptake of substances across the membrane.

The other terms mentioned do not describe this specific structure. Cilia and flagella are motile cell projections involved in movement, and macrovilli is not a standard biological term used to describe any cellular structures. Hence, microvilli are the correct and specific term for those small, finger-like projections of the plasma membrane contributing to the brush border.