This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Miscellaneous (Structure & Function of Proteins), Subject name: Biochemistry
Precursor of norepinephrine - 
Select and explain. 
A. Tryptophan, B. Tyrosine, C. Methionine, D. Asparagine
B: Tyrosine
Answer: B: Tyrosine 
Explanation: Ans. is 'b' i.e.. Tyrosine