Identifying an Essential Amino Acid

In the study of biochemistry, amino acids are the building blocks of proteins. These organic compounds contain both an amino group and a carboxyl group, along with a side chain (R group) that determines the identity of the amino acid. Each amino acid can be categorized into essential and non-essential amino acids. Essential amino acids cannot be synthesized by the body and, thus, must be obtained from the diet.

Which of the following amino acids is classified as essential?