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
Biuret reaction is specific for 
Select and explain. 
A. –CONH-linkages, B. –CSNH2 group, C. –(NH)NH2 group, D. All of these
A: –CONH-linkages
Answer: A: –CONH-linkages 
Explanation: Biuret Reaction : Cupric ions in an alkaline medium form a violet color with peptide bond nitrogen (Schiff, 1896). This needs a minimum of two peptide bonds, and so individual amino acids and dipeptides will not answer this test.
Reference: Vasudevan 7th  ed, pg 31