Congenital lactase deficiency

**Definition:** This is a congenital deficiency of lactase (EC 3.2.1.108), inherited as an autosomal recessive trait, presenting in infancy and manifested by profuse watery diarrhoea in response to dietary milk, due to inability to digest lactose, a sugar found in milk and to a lesser extent milk-derived dairy products. The condition may lead to marasmus and death if lactose is not eliminated from the diet.

**Long definition:** This is the congenital inability to digest lactose, a sugar found in milk and to a lesser extent milk-derived dairy products.