This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Methionine and Cysteine details, Subject name: Biochemistry
Homocysteine is 
Select and explain. 
A. Amino-acid in Protein- D form, B. Amino-acid in Protein- L form, C. Non-protein alpha amino-acid, D. Seen in Collagen
C: Non-protein alpha amino-acid
Answer: C: Non-protein alpha amino-acid 
Explanation: Homocysteine is a non-proteinogenic alpha- amino acid Homologous to cysteine, differing by an additional methylene bridge (-CH2-) Synthesized from methionine.