This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Biochemistry
Which among the following amino acid is heterocyclic in structure? 
Select and explain. 
A. Histidine, B. Phenylalanine, C. Hydroxyproline, D. Proline
A: Histidine
Answer: A: Histidine 
Explanation: Histidine and tryptophan are heterocyclic amino acids. Phenylalanine and tyrosine are aromatic amino acids. Proline is an imino acid. Hydroxyproline, hydroxylysine, and ornithine are derived amino acids. Ref: Textbook of Biochemistry for Medical Students By Dm Vasudevan, 2010, Page 20.