This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Metabolism of protein and amino acid, Subject name: Biochemistry
Cystine is formed from 
Select and explain. 
A. Arginine, B. Histidine, C. Cysteine, D. Alanine
C: Cysteine
Answer: C: Cysteine 
Explanation: The difference in structure of cysteine and cystine. Two molecules of cysteine are joined together by S--S bond to form one molecule of cystine.Ref: M.N. Chatterjea - Textbook of Biochemistry, 8th edition, page no: 504