Infants with Phenylketonuria (PKU) appear completely normal at birth. However, if not diagnosed and managed early, they may develop symptoms such as feeding difficulties, vomiting, and skin rashes in the subsequent months. Untreated PKU leads to an accumulation of phenylalanine in the brain, which can cause severe neurological complications and intellectual disability; this often results in IQ levels dropping below 50. Due to the interaction of phenylalanine with melanin pigment, affected infants typically have fair skin, light hair, and blue eyes.

The signs and symptoms of Phenylketonuria, which can range from mild to severe, include:
* A distinctive musty odor in the breath, skin, or urine.
* Seizures and other neurological problems.
* Skin rashes, such as eczema.
* Abnormally small head (microcephaly).
* Hyperactivity.
* Delayed development.
* Behavioral, emotional, and social problems.
* Severe intellectual disability.
* Psychiatric disorders.