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
Which of the following drug is a direct inhibitor of clotting factor Xa? 
Select and explain. 
A. Apixaban, B. Argatroban, C. Fondaparinux, D. Aspirin
A: Apixaban
Answer: A: Apixaban 
Explanation: Impoant ones are: - (1) Rivaroxaban (2) Apixaban (3) Edoxaban (4) Betrixaban (a) Argatroban - Direct thrombin inhibitor (synthetic non-peptide compound which binds reversibly to the catalytic site of thrombin but not to the substrate recognition site) (b) Fondaparinux - Indirect thrombin inhibitor. (d)Aspirin - Inhibit COX and thus Thromboxane A2 synthesis inhibitor