Which enzyme is responsible for the conversion of inactive fibrinogens to fibrins?

**Correct Answer:** Thrombin

**Step‑by‑Step Explanation**

1. Blood clotting basics: When a blood vessel is injured, the body must form a stable clot to stop bleeding. The clot is primarily made of fibrin, a fibrous protein that creates a meshwork trapping blood cells.

2. Fibrinogen: Fibrinogen is a soluble plasma protein that circulates in the blood in an inactive form. It is the precursor that will become fibrin once activated.

3. Enzymatic conversion: The conversion of fibrinogen to fibrin is a proteolytic cleavage reaction. A specific enzyme cleaves fibrinopeptides A and B from fibrinogen, allowing the remaining molecules to polymerize into insoluble fibrin strands.

4. Role of thrombin: The enzyme that carries out this cleavage is thrombin. Thrombin is generated from its inactive precursor prothrombin in the final steps of the coagulation cascade, which is triggered by tissue factor exposure and platelet activation.

5. Outcome: Once thrombin acts on fibrinogen, the released fibrinopeptides expose polymerization sites, and the resulting fibrin monomers quickly assemble into a stable fibrin clot, sealing the wound.

Therefore, the enzyme responsible for converting inactive fibrinogen to fibrin is **thrombin**.