rare genetic condition known as phenylketonuria ((fen-ul-keetone-YU-ree-ah), PKU or Folling disease, is an inherited metabolic
disorder caused by both parents containing a PKU gene that is transferred to
their offspring. Phenylketonuria is a disorder in which the body is unable to
break down the amino acid, phenylalanine (PHE).

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The vital enzyme required for the protein to be processed is called phenylalanine
hydroxylase. PKU is caused
by a defect in the gene that creates phenylalanine hydroxylase, which is the
enzyme necessary for building neurotransmitters (chemical messengers). In this case, it is deficient in the individual with
the disease, causing an excess amount of the protein in the body cells. This can instigate nervous system or brain damage in a
person. There are different levels of severity for the phenylketonuria
disease. The scale goes from mild to severe, the worst case being the “classic
PKU”. This genetic disease was discovered by Dr. Asbjørn Følling in Norway, when he
tested the urine samples of two children who were mentally challenged. Folling
noticed, after careful examination and many tests, their urine contained
phenylpyruvic acid. He wrote about his scientific discovery in the journal Hoppe
Seylers Zeitschrift fur Physiologische Chemie, in 1934 saying, “On excretion of
phenylpyruvic acid in the urine as an anomaly of metabolism in connection with
mental retardation.”