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  1. Due to deficiency of phenylalanine hydroxylase, therefore phenylalanine cannot be converted to tyrosine.
  2. Accessory metabolic pathways start operating which convert phenylalanine into phenyl pyruvic acid, phenyl lactic acid, and phenylacetic acid. Q
  3. These metabolic and phenylalanine disrupts normal metabolism and causes brain damage. 

Clinical Features

  1. The child is usually normal at birth. Q
  2. Neurological signs such as irritability, tremors, convulsions, hyperkinesis and muscular hypertonia manifests after some weeks, about 25% children have seizures.
  3. Vomiting severe enough to be misdiagnosed as pyloric stenosis. Q
  4. High levels of phenylalanine results in competitive inhibition of tyrosinase, which converts tyrosine into melanin. This accounts for blond hair, blue iris and fair skin. Some patients may have a seborrheic or eczematoid skin rash which is usually mild and disappear as the child grows older. Q
  5. These children have an unpleasant odor, which has been described as musty or mousey. Q
  6. Microcephaly, prominent maxilla with widely spaced teeth, enamel hypoplasia and growth retardation are other common finding in untreated children. Q

Fig: Approach to a newborn infant with suspected metabolic defect. NH3 ammonia, PKU phenylketonuria, NKH nonketotic hyperglycinemia.


  1. Phenylalanine level > 20 mg %: Classical phenylketonuria. Q
  2. Normal plasma tyrosine level
  3. Increased urinary level of metabolites of phenylalanine. 

For Screening

  1. Guthrie’s test – It is a microbiological test based on the principle that phenylalanine is necessary for growth of Bacillus subtills.
  2. FeCl3 Test: PKU patients urine + Fecl3 :- Green color. Q 



Phenylalanine should be restricted in the Infants.


Controlled dietary restriction started at birth and continued for whole life. Q

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