Below is an instruction from Human. Write a response.
    ### Instruction:
    What is (are) lactose intolerance ?
    ### Response:
    Lactose intolerance is an impaired ability to digest lactose, a sugar found in milk and other dairy products. Lactose is normally broken down by an enzyme called lactase, which is produced by cells in the lining of the small intestine.  Congenital lactase deficiency, also called congenital alactasia, is a disorder in which infants are unable to break down lactose in breast milk or formula.
    