Fibrin is best described as what?

• A. A hormone
• B. A protein that strengthens a blood clot
• C. A carbohydrate
• D. A lipid

Answer: (B)

Fibrin is a protein that forms the fibrous mesh that strengthens a blood clot. During coagulation, soluble fibrinogen in blood is converted into insoluble fibrin by the enzyme thrombin. The resulting fibrin strands weave together with platelets to trap cells and create a durable clot. This distinguishes fibrin from hormones (signaling molecules), carbohydrates (sugars/polysaccharides), and lipids (fats), since those are different types of molecules with other roles.