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 is the amino acid shown below 
Select and explain. 
A. Histidine, B. Proline, C. Phenylalanine, D. Tyrosine
B: Proline
Answer: B: Proline 
Explanation: Proline is an imino acid, because it has an imino group (=NH) instead of amino group (-NH2).
The ring structure present in proline in known as pyrrolidine ring.