Which of the following aromatic amino acids is classified as nonessential in humans?

• A. Tryptophan
• B. Phenylalanine
• C. Histidine
• D. Tyrosine

Answer: (D)

Verified step by step guidance

1. Step 1: Begin by understanding the classification of amino acids. Amino acids are categorized as essential or nonessential based on whether the human body can synthesize them. Essential amino acids must be obtained through the diet, while nonessential amino acids can be synthesized by the body.
2. Step 2: Review the aromatic amino acids. Aromatic amino acids contain a side chain with an aromatic ring structure. The primary aromatic amino acids in humans are phenylalanine, tyrosine, tryptophan, and histidine.
3. Step 3: Recall the metabolic relationship between phenylalanine and tyrosine. Tyrosine is synthesized in the human body from phenylalanine through the action of the enzyme phenylalanine hydroxylase. This makes tyrosine a nonessential amino acid, as it can be produced internally.
4. Step 4: Compare the other options. Tryptophan, phenylalanine, and histidine are essential amino acids because the human body cannot synthesize them and they must be obtained through dietary sources.
5. Step 5: Conclude that tyrosine is the correct answer, as it is classified as a nonessential aromatic amino acid in humans due to its synthesis from phenylalanine.