Symptoms of phenylketonuria (PKU) usually develop within a few months after birth, after phenylalanine has built up in a baby's system from consuming the protein in formula or breast milk. Before birth, the mother's body filters out the excess phenylalanine for the baby (fetus).

Early symptoms of PKU, which occur in more than half of babies with this condition, include:³

• A musty odor to the skin, hair, and urine.
• Vomiting and diarrhea, leading to weight loss.
• Irritability.
• Skin problems, such as dry skin, or itchy skin rashes (eczema).
• Sensitivity to light (photosensitivity).

If a baby has especially low levels of the enzyme needed to prevent phenylalanine build-up or if PKU is not detected and treated soon enough, phenylalanine builds up in the brain tissue. It then affects mental skills and the central nervous system. Symptoms can become severe by about 8 weeks of age and may include:

• Unusual behavior, such as screaming episodes, repetitive rocking, head banging, and arm biting (common in older children).
• Loss of skills and abilities related to severe mental retardation.
• Growth and developmental delays.
• Seizures.

PKU also affects the synthesis of melanin, which provides pigment (color) to the skin, eyes, and hair. About 90% of children with PKU have blond hair, fair skin, and blue eyes.³