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: Surgery
A patient who is on dicoumoral therapy require a tooth extraction. Which laboratory test is the most valuable in evaluating this patient's surgical risk? 
Select and explain. 
A. Clotting time, B. Bleeding time, C. Prothrombin time, D. Complete blood cell count
C: Prothrombin time
Answer: C: Prothrombin time 
Explanation: None