Which of the following is essential for blood clotting?

• A. Osmotic pressure maintenance.
• B. Oxygen transport.
• C. Fibrinogen.
• D. Immune defense.

Answer: (C)

Clot formation hinges on making a stable fibrin mesh. Fibrinogen is a soluble plasma protein that, when activated by the enzyme thrombin, is cleaved to become insoluble fibrin. These fibrin strands weave through the platelet plug to form a durable blood clot that seals the vessel. Without fibrinogen, the final clot cannot develop, even if platelets are present.

The other options relate to different physiological roles: osmotic balance helps regulate fluid distribution, oxygen transport depends on hemoglobin in red blood cells, and immune defense involves white blood cells and antibodies. None of these are the key step that forms the clot’s structural framework.