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
Which of the follwoing is a source of norepinephrine? 
Select and explain. 
A. Phenylalanine, B. Alanine, C. Tyrosine, D. Tryptophan
C: Tyrosine
Answer: C: Tyrosine 
Explanation: 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. Ref: Textbook Of Medical Physiology By Khurana, 2006, Page 769; Lehninger Principles of Biochemistry, 4th Edition, Page 440, 678, 859