Classic Phenylketonuria (PKU)



Classic phenylketonuria (PKU) is a condition in which the body is unable to break down certain building blocks of proteins, known as amino acids. Individuals with PKU cannot break down the amino acid phenylalanine. If left untreated, PKU can cause brain damage or even death. However, if the condition is detected early and treatment is begun, individuals with PKU can lead healthy lives. Phenylketonuria is a condition with multiple forms, each with different treatments and outcomes. Classic PKU is the most severe form. Babies with PKU usually seem healthy at birth, but signs of the condition begin to appear around 6 months of age. Signs of PKU include irritability, seizures (epilepsy), dry, scaly skin (eczema), musty body odor, pale hair and skin, and developmental delays. Many of these signs may occur when a baby eats foods the body cannot break down. 

Condition Type:

Core Conditions


In the United States, 1 in every 10,000 to 15,000 babies is affected by phenylketonuria (PKU).

More Information for Parents:
Also known as:
  • Phenylketonuria
  • Classic phenylketonuria
  • Phenylalanine hydroxylase deficiency
  • Hyperphenylalaninemia — classic type
  • PAH deficiency

Core Conditions