Canadian Society for Medical Laboratory Science (CSMLS) Practice Exam

Which of the following is not a plasma protein?

1. Fibrin

2. Albumin

3. Alpha globulin

4. Fibrinogen

The correct answer is: Fibrin

Fibrin is not classified as a plasma protein because it is actually a product of fibrinogen, which is a plasma protein that plays a crucial role in blood clotting. When bleeding occurs, fibrinogen is converted into fibrin through the action of the enzyme thrombin. This transformation is essential for forming a stable blood clot. In contrast, fibrinogen itself is a soluble plasma protein, while fibrin, being the insoluble fibrous form, is part of the clot that forms in response to injury and is not typically found in the plasma under normal conditions. Albumin, alpha globulin, and fibrinogen are all examples of plasma proteins. Albumin is the most abundant plasma protein and is important for maintaining osmotic pressure and transporting various substances in the blood. Alpha globulins refer to a group of proteins that have various functions, including acting as enzymes or transport proteins. Fibrinogen, as mentioned, is crucial for coagulation. Understanding these distinctions will clarify the roles various proteins play in the circulatory system and their importance in both health and disease contexts.