This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Hematology, Subject name: Pharmacology
What is the mode of action of warfarin? 
Select and explain. 
A. Factor Xa inhibitor, B. Vitamin K antagonist, C. Activates antithrombin III, D. Activates factor IX
B: Vitamin K antagonist
Answer: B: Vitamin K antagonist 
Explanation: Ref: KD Tripathi, 7th ed. pg. 620, 621* Warfarin and its congeners act as anticoagulants only in vivo (body), NOT in vitro (lab).* It acts by inhibiting Vitamin K. Due to overdose, hematuria is the first manifestation noted.* Dose monitoring is done by INR* Antidote of warfarin overdose: Vitamin K