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
Phenylalanine is the precursor of all the following except 
Select and explain. 
A. Tyrosine, B. Epinephrine, C. Thyroxine, D. Melatonin
D: Melatonin
Answer: D: Melatonin 
Explanation: None