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: Pharmacology
Heparin inhibits Xa by 
Select and explain. 
A. Blocking vitamin K mediated activation of Xa, B. Activating Antithrombin III, C. Direct breakdown of Xa, D. Blocking P2 Y12 receptor of ADP
B: Activating Antithrombin III
Answer: B: Activating Antithrombin III 
Explanation: Heparin contains pentasaccharide unit that activates Antithrombin III & inhibits factor Xa (Thus it is a indirect thrombin inhibitor).
Unfractionised heparin also contains a heteropolysaccharide unit that inhibits factor II a.
LMWH lacks this heteropolysacchoride unit and therefore inhibitors factor Xa only.