Which of the following is an essential amino acid?
(a) Cystine (b) Glycine
(c) Phenylalanine (d) Serine
(c) Essential amino acids are amino acids that must be included in the diet of higher vertebrates because they cannot be synthesized by these organisms. The amino acids essential in the human diet are isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan and valine.