Which vitamin's deficiency is associated with prolonged prothrombin time (PT)?

A deficiency in vitamin K is associated with a prolonged prothrombin time (PT). Vitamin K plays a crucial role in the synthesis of certain proteins known as clotting factors, which are essential for blood coagulation. The liver produces several clotting factors that rely on vitamin K for their synthesis, particularly factors II (prothrombin), VII, IX, and X. When there is an inadequate amount of vitamin K in the body, the production of these factors is impaired, leading to an increased prothrombin time. As prothrombin time is used as a clinical measure to assess the blood's ability to clot properly, a prolongation indicates a heightened risk of bleeding due to inadequate clotting factor levels.

Other vitamins mentioned do not directly affect prothrombin time. For instance, vitamin A deficiency does not influence clotting factors, vitamin B12 is primarily associated with red blood cell production and neurological function, and vitamin D is mostly involved in calcium metabolism and bone health. Therefore, vitamin K's specific role in clotting factor synthesis makes it the correct answer to the question regarding prolonged prothrombin time.