Below is an instruction from Human. Write a response.
    ### Instruction:
    What is (are) phenylketonuria ?
    ### Response:
    Phenylketonuria (commonly known as PKU) is an inherited disorder that increases the levels of a substance called phenylalanine in the blood. Phenylalanine is a building block of proteins (an amino acid) that is obtained through the diet. It is found in all proteins and in some artificial sweeteners.
    