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: Biochemistry
Source of norepinephrine is? 
Select and explain. 
A. Phenylalanine, B. Alanine, C. Tyrosine, D. Tryptophan
C: Tyrosine
Answer: C: Tyrosine 
Explanation: Tyrosine REF: Lehninger Principles of Biochemistry 4th Edition page 440, 678, and 859 "Tyrosine gives rise to a family of catecholamines that includes dopamine, norepinephrine, and epinephrine" "Norepinephrine is synthesized by a series of enzymatic steps in the adrenal medulla and postganglionic neurons of the sympathetic nervous system from the amino acid tyrosine"