This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Miscellaneous (Bio-Chemistry), Subject name: Biochemistry
Which one of the following is a heterocyclic amino acid? 
Select and explain. 
A. Leucine, B. Methionine, C. Alanine, D. Proline
D: Proline
Answer: D: Proline 
Explanation: (D) Proline # Proline is one of the nonessential amino acids, found in many proteins, especially collagen.> Because the nitrogen atom of its amino group is part of a ring structure (making it a heterocyclic compound), its chemical properties differ from those of the other amino acids in proteins.> It is used in biochemical, nutritional, and microbiological research and as a dietary supplement.