This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Physiology
Which of the following agents in not likely to be found in plasma? 
Select and explain. 
A. Thrombin, B. Fibrinogen, C. Prothrombin, D. Calcium ion
A: Thrombin
Answer: A: Thrombin 
Explanation: None