Which substance is classified as a first-generation anticoagulant?

Warfarin is classified as a first-generation anticoagulant because it functions by inhibiting the synthesis of vitamin K-dependent clotting factors in the liver, which are essential for normal blood coagulation. This results in a prolonged clotting time and helps prevent blood clots from forming. First-generation anticoagulants like Warfarin have been widely used in both veterinary and human medicine for several decades.

In contrast, cholecalciferol is primarily a vitamin D3 analog and works by causing hypercalcemia, rather than acting as an anticoagulant. Bromethalin is a neurotoxic rodenticide that functions through a different mechanism affecting the nervous system. Zinc phosphide is a rodenticide that releases phosphine gas upon contact with stomach acid, leading to poisoning, but it does not have anticoagulant properties like Warfarin. Thus, Warfarin stands out as the correct answer due to its specific action in disrupting the blood clotting process.