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 is done for a patient on coumarin (warfarin) therapy: 
Select and explain. 
A. Partial thromboplastin time (PTT), B. Prothrombin time (PT), C. Bleeding time (BT), D. Capillary fragility test (CFT)
B: Prothrombin time (PT)
Answer: B: Prothrombin time (PT) 
Explanation: When a coumarin, such as warfarin, is given to a patient, the amounts of active prothrombin and Factors VII, IX, and X, all  formed by the liver, begin to fall. 
Prothrombin time gives an indication of the concentra­tion of prothrombin in the blood.
The normal prothrombin time is about 12 seconds.
Reference:  Ganong Physiology 25th ed page no 492,493