This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Classification and metabolism of amino acids, Subject name: Biochemistry
Precursor of tyrosine is: 
Select and explain. 
A. Cysteine, B. Histidine, C. Tryptophan, D. Phenylalanine
D: Phenylalanine
Answer: D: Phenylalanine 
Explanation: Phenylalanine in the presence of phenylalanine hydroxylase forms tyrosine . So precursor for tyrosine is phenylalanine Extra Information: Enzyme Disease Phenylalanine hydroxylase Classical phenylketonuria or PKU-1 Dihydrobiopterin reductase Non-classical PKU type 2 & 3 GTP-Cyclo hydrolase Non-classical type 4 or 5