This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Metabolism of protein and amino acid, Subject name: Biochemistry
The rate-limiting step for norepinephrine synthesis 
Select and explain. 
A. Conversion of phenylalanine to Tyrosine, B. Conversion of Tyrosine to DOPA, C. Conversion of DOPA to dopamine, D. Conversion of dopamine to norepinephrine
B: Conversion of Tyrosine to DOPA
Answer: B: Conversion of Tyrosine to DOPA 
Explanation: In the synthesis of Norepinephrine, Tyrosine is hydroxylated to 3,4dihydroxyphenylalanine by tyrosine hydroxylase. This enzyme catalyses the rate limiting reaction and require tetrahydrobiopterin as coenzyme.Ref: Biochemistry, U. Satyanarayana, Fouh edition, page no: 349