is short for hyperphenylalaninemia, a genetic illness, which a child inherits from both parents, that leaves the child unable to digest particular proteins. Treatment in the form of a special diet must be started in the first few weeks of life to prevent mental retardation. For more on PKU visit the Montreal Children's Hospital site.

PKU is a rare hereditary condition involving the inability to metabolize the amino acid phenylalanine (Phe). Affected individuals must adhere to a diet low in Phe during childhood or Phe levels rise and become toxic, causing central nervous system injury. Women with PKU must also adhere to a low Phe diet before and during pregnancy to avoid fetal damage. Newborns are routinely screened for PKU, which can cause mental retardation if not detected early.