This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Pediatrics
A child brought with seizures and impairment of development is diagnosed having phenylketonuria. What is the initial line of treatment in this child? 
Select and explain. 
A. Dietary supply of deficient protein, B. Restriction of substrate of the deficient enzyme in the diet, C. Replacing the products, D. Dietary supply of deficient pro enzyme
B: Restriction of substrate of the deficient enzyme in the diet
Answer: B: Restriction of substrate of the deficient enzyme in the diet 
Explanation: Phenylketonuria: It is an autosomal recessive metabolic disorder, where the level of hepatic enzyme phenylalanine hydroxylase is low, therefore phenylalanine cannot be conveed to tyrosine. Phenylalanine accumulates in the blood, cerebrospinal fluid and tissues. Accessory metabolic pathways sta operating which conve phenylalanine to phenylpyruvic acid, phenyl-lactic acid and o-hydroxyphenyl-acetic acid. Restriction of phenylalanine in the diet is the mainstay of treatment of phenylketonuria. Ref: Essential Pediatrics by O.P. Ghai, 6th edition, Page 609.