One year old male child with sparse blond hair, developmental delay and tremors, diagnosis

Phenylketonuria is due to deficiency of enzyme phenylalanine hydroxylase or of its cofactor tetrahydrobiopterin ;leads to accumulation of phenylalanine in body fluids and brain. It is mainly characterized by profound mental retardation ,lighter complexion and neurologic symptoms like tremors,hyperreflexia and spasticity. Reference: Nelson TB of pediatrics 19th edition, pg 418