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

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

Answer: (C)

Vitamin K is produced by gut bacteria and is essential for coagulation. It acts as a cofactor for the enzyme that gamma-carboxylates glutamate residues on key clotting factors (II, VII, IX, X) and proteins C and S, enabling these factors to bind calcium and participate effectively in the clotting cascade. Because gut flora synthesize this vitamin, disruptions to the gut microbiome—such as from antibiotics—can transiently lower vitamin K production and affect coagulation. This is why newborns receive vitamin K prophylaxis at birth, and why vitamin K status is closely linked to bleeding risk measured by coagulation tests like the prothrombin time. The other vitamins listed do not have a primary production role in the gut related to coagulation: they serve different roles (antioxidant/collagen formation for vitamin C, calcium regulation for vitamin D, and vision/cell health for vitamin A).