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
Essential amino acid is: 
Select and explain. 
A. Phenylalanine, B. Proline, C. Glutamine, D. Serine
A: Phenylalanine
Answer: A: Phenylalanine 
Explanation: Phenylalanine It is an alpha-amino acid. This essential amino acid is classified as nonpolar because of the hydrophobic nature of the benzyl side chain. L-Phenylalanine (LPA) is an electrically neutral amino acid The codons for L-phenylalanine are UUU and UUC. Phenylalanine is a precursor for tyrosine, the monoamine signaling molecules dopamine, norepinephrine (noradrenaline), and epinephrine (adrenaline), and the skin pigment melanin.