What substance was discovered through testing on cats that helps prevent blood clotting?

The substance identified through testing on cats that assists in preventing blood clotting is anticoagulants. These compounds work by inhibiting certain clotting factors in the blood, ultimately reducing the ability for clots to form. Anticoagulants play a crucial role in medicine, particularly for conditions where blood clots pose a significant risk, such as thrombosis or after certain surgeries.

In contrast, analgesics are pain relievers that do not affect blood clotting mechanisms. Vitamins are organic compounds essential for various bodily functions, but do not function specifically to prevent blood clotting. Antibiotics are medications used to treat bacterial infections and do not have any direct effect on the blood coagulation process. This differentiation highlights why anticoagulants are the correct choice in the context of preventing blood clots.