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
Strength and rigidity in keratin is due to - 
Select and explain. 
A. Leucine, B. Cysteine, C. Lithium, D. None of the above
B: Cysteine
Answer: B: Cysteine 
Explanation: Keratin is a protein present in the hair, nails and outer layer of skin. Keratin is almost completely comprised of alpha-helices (secondary structure) Two alpha helix are wound around one another to form super helix. Strength and rigidity in keratin is due to Cysteine because of its ability to form disulphide bonds. Number of disulphide bonds is directly propoional to rigidity of keratin i.e. more the disulphide bonds, harder the keratin.