A. Slowing of blood (atrial fibrillation, congestive heart failure, heart attack, bed rest, paralysis). B. Abnormal surfaces in contact with blood (vascular injury, heart valve replacement or disease, atherosclerosis). C. Abnormalities in endogenous anti-clotting factors (protein C and S deficiency, antithrombin). D. Estrogen therapy. E. All of the above.
The answer is all of the above. When blood slows down, as answer A suggests, the blood can stagnate and become clotted. The clot happens because the blood can’t move, but also because blood likes to stick to itself - that’s how a clot is formed to begin with. This is often combated in hospitals with special pumping tourniquets that will keep the blood from stagnating.
An abnormal surface in contact with blood can also cause clotting. It gives the blood something else to stick to, and gives it a place to start the clot. These can be dangerous if the cause isn’t identified.
An abnormality in the blood clotting systems can also cause clots to happen by allowing the blood to clot too easily. Estrogen therapy is another cause that can create clots that shouldn’t be there. It simply increases the chances that a clot will form where it shouldn’t.