Which of the following vitamins is mainly responsible for blood coagulation in human being?

The correct answer is A) Vitamin K. Vitamin K is essential for the liver to synthesise several clotting factors, namely II (prothrombin), VII, IX and X, along with proteins C and S. These factors need vitamin K for the gamma-carboxylation of glutamate residues, without which they cannot bind calcium and take part in the clotting cascade. This is why deficiency leads to prolonged bleeding and why newborns are given a vitamin K injection at birth.

Why the others are wrong:

• Vitamin E is an antioxidant that protects cell membranes from lipid peroxidation. It has no major role in coagulation, and high doses can slightly impair platelet function, which is a common confusion.
• Vitamin A is needed for vision (rhodopsin), epithelial health and immunity. It is not a clotting vitamin.
• Vitamin C is needed for collagen synthesis through hydroxylation of proline and lysine. Its deficiency causes scurvy with bleeding gums, but it does not directly drive coagulation.

Conceptually, this sits under the nutrition and biochemistry segment of Science, and it links to Mains GS-III on health and nutrition, especially deficiency disorders and the role of micronutrients in public health.

Trap: Aspirants pick Vitamin C because of scurvy's bleeding symptoms. Remember that scurvy is a collagen defect, while the coagulation vitamin is K.