Which structural feature differentiates tryptophan from phenylalanine and tyrosine?

• A. Tryptophan has a five-membered ring with a nitrogen atom.
• B. Tryptophan has a sulfur atom in its side chain.
• C. Tryptophan has a hydroxyl group attached to the benzene ring.
• D. Tryptophan has an additional methyl group.

Answer: (A)