The inability to digest or absorb lactose, a type of sugar found in milk and other dairy products.
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Congenital lactase deficiency is an autosomal recessive disorder affecting newborns and characterized by severe, watery diarrhea. The symptoms manifest within the first days of life, soon after the first lactose containing meal. The newborn fails to gain weight due to malabsorption of lactose. The symptoms disappear after introduction of a lactose-free diet.