What protein breaks down to form D-dimer during clot dissolution?

D-dimer is a small protein fragment present in the blood after a blood clot dissolves. It is formed specifically from the breakdown of fibrin, which is one of the key proteins involved in the clotting process. When a blood clot is formed, fibrin strands weave through the platelets, providing structure and stability to the clot. During the process of fibrinolysis, the enzyme plasmin dissolves fibrin, leading to the creation of D-dimer as the fibrin is degraded.

Fibrinogen, while an important protein in the formation of fibrin, does not directly lead to D-dimer formation. Thrombin and prothrombin are crucial in the coagulation cascade for clot formation but do not play a direct role in the breakdown of clots or the formation of D-dimer. Thus, fibrin is indeed the protein that breaks down to produce D-dimer during the process of clot dissolution, confirming the correctness of the answer.