Which brush border enzyme is lacking in an individual who is lactose intolerant?

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The enzyme missing in lactose intolerance is lactase, the brush border disaccharidase that hydrolyzes lactose into glucose and galactose for absorption in the small intestine. Without lactase, lactose remains undigested, drawing water into the intestine and undergoing fermentation by colonic bacteria, which leads to symptoms like gas, bloating, and diarrhea. Other brush border enzymes—maltase, sucrase, and trehalase—digest different disaccharides (maltose, sucrose, and trehalose, respectively) and are not specifically deficient in lactose intolerance, so their presence or absence doesn’t explain the problem. In many lactose-intolerant individuals, lactase activity declines with age, a condition known as lactase non-persistence. Therefore, lactase is the enzyme lacking.