This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Phenylalanine and Tyrosine metabolism Detail, Subject name: Biochemistry
In Phenylketonuria, the first line therapy is: 
Select and explain. 
A. Replacement of the defective enzyme, B. Replacement of the deficient product, C. Limiting the substrate for deficient enzyme, D. Giving the missing amino acid by diet
C: Limiting the substrate for deficient enzyme
Answer: C: Limiting the substrate for deficient enzyme 
Explanation: In phenylketonuria:- There is accumulation of phenylalanine Accumulated phenylalanine inhibits the transpo of other amino acids required for neurotransmitter synthesis, reduces synthesis and increases degradation of myelin - Mental retardation Breastfeeding introduces phenylalanine and symptoms will be:- 1. Vomiting 2. Irritability 3. Convulsions & tremors are the usual presentations. Thus, limiting the substrate of the deficient enzyme, i.e. phenylalanine, is the first-line of management. Breastfeeding should be avoided.