This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Structure and function of protein, Subject name: Biochemistry
Which of the following is a modified amino acid? 
Select and explain. 
A. Arginine, B. Cysteine, C. Asparagine, D. Threonine
B: Cysteine
Answer: B: Cysteine 
Explanation: Two molecules of cysteine make cysteine (cys-cys) or dithio b, b-a amino propionic acid. The SS linkage is called as disulfide bridge. Ref: Textbook of medical biochemistry, MN Chatterji, 8th edition, page no: 78