Identifying an Essential Amino Acid

Amino acids are the building blocks of proteins and play essential roles in various biological processes. Each amino acid consists of a central carbon atom, an amino group (-NH₂), a carboxyl group (-COOH), a hydrogen atom, and a variable R group (side chain) that determines its unique properties. Out of the 20 standard amino acids, some are classified as essential, meaning they cannot be synthesized by the body and must be obtained through the diet.

Which of the following statements correctly identifies an essential amino acid?