Ans: B. White patch of hair due to tryptophan deficiencyThe deficiency of phenylalanine hydroxylase is the cause of this disease.The genetic mutation may be such that either the enzyme is not synthesized, or a non-functional enzyme is synthesized.The classical Phenylketonuria child is mentally retarded with an IQ of 50.Guthrie test is a rapid screening test.Ferric chloride test: The urine of the patient contains phenyl ketones about 500-3000 mg/day.This could be detected by adding a drop of ferric chloride to the urine.A transient blue-green colour is a positive test.