Below is an instruction from Human. Write a response.
    ### Instruction:
    What is (are) Tetrahydrobiopterin deficiency ?
    ### Response:
    Tetrahydrobiopterin (BH4) deficiency is a neurological condition caused by an inborn error of metabolism. BH4 is a substance in the body that enhances the action of other enzymes. Deficiency of BH4 leads to abnormally high blood levels of the amino acid phenylalanine, and low levels of certain neurotransmitters.
    