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
Keratin is present in both skin and nail. However, skin is much more flexible than the nail. This is because keratin of skin has? 
Select and explain. 
A. Less number of disulfide bonds, B. Lesser number of salt bridges, C. Higher Na content, D. Different affinities of water
A: Less number of disulfide bonds
Answer: A: Less number of disulfide bonds 
Explanation: Keratins are fibrous proteins present in hair, skin and nails, horn, hoof, etc. They mainly have the alpha helical structure. Each fibril has 3 polypeptide chains and each bundle has about 10-12 bundles. The matrix has cysteine-rich polypeptide chains which are held together by disulfide bonds. The more the number of disulfide bonds, the harder the keratin is. This is the reason behind skin is more flexible than nail. Ref: Essentials of Medical Biochemistry: With Clinical Cases By N. V. Bhagavan, Chung-Eun Ha, 2011, Page 33