Which vitamin is essential for coagulation and is produced by gut bacteria?

• A. Vitamin C
• B. Vitamin K
• C. Vitamin D
• D. Vitamin A

Answer: (B)

Vitamin K is needed to activate clotting factors in the coagulation cascade by gamma-carboxylating glutamate residues, which lets these factors bind calcium and work properly. A unique aspect is that gut bacteria produce a form called menaquinone (vitamin K2), contributing to the body's supply. This combination explains why vitamin K is essential for coagulation and why it’s produced by gut flora. The other vitamins listed do not play this coagulation role and are not produced by gut bacteria to support clotting. Clinically, a deficiency leads to increased bleeding risk, and newborns receive vitamin K prophylaxis because their gut flora isn’t yet established.