An infant has eczematous rash, rhythmic rocking microsephaly and hypeonia. What is the likely diagnosis ?

Phenylketonuria is a disorder of phenylalanine metabolism and occurs due to deficiency of phenylalanine hydroxylase.Affected individuals have profound and irreversible intellectual disability,mental retardation,microcephaly,epilepsy & behavioral problems.These patients often have a musty body odour and skin conditions such as eczema caused by excretion of excessive phenylalanine and its metabolites.Decreased skin,hair,nail pigmentation may also occur due to associated inhibition of tyrosinase and reduced Melanin synthesis Reference:Essential pediatrics-Ghai,8th edition,page no:652