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
Collagen protein axial periodicity is - 
Select and explain. 
A. 67nm, B. l07nm, C. 207nm, D. 307nm
A: 67nm
Answer: A: 67nm 
Explanation: Ans. is 'a' i.e., 67 nm.The fibrillogenesis involves the lateral association of collagen triple helices, the structural unit of collagen, to form long, smooth fibrils with a characteristic 67-nm axial structural feature known as the D-periodicity or axial periodicity.