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Rugae are the internal folds of the stomach lining that allow the stomach to expand as it fills with food and liquid. This feature is crucial for accommodating varying volumes of ingested material. As the stomach fills, rugae flatten out to create more surface area, facilitating the mechanical processing of food without allowing for absorption, which primarily occurs later in the digestive tract.

In contrast, villi and microvilli are structures found in the small intestine and are essential for nutrient absorption. Villi are the larger, finger-like projections that increase the surface area of the intestinal lining, while microvilli are even smaller projections on the epithelial cells of the villi that further enhance absorption efficiency. The brush border refers specifically to the collective microvilli on the intestinal cells that contribute to the absorption process. Since their primary function is absorption, they do not pertain to the stomach's ability to expand.

Therefore, rugae serve a unique role in the stomach, distinct from the absorption functions associated with villi, microvilli, and the brush border.