**Core Concept**
Fondaparinux is a synthetic anticoagulant that selectively inhibits the coagulation pathway by targeting Factor Xa, a critical enzyme in the formation of thrombin and subsequent clot formation. This selective inhibition of Factor Xa leads to a reduction in thrombin generation and subsequent clot formation.

**Why the Correct Answer is Right**
Fondaparinux works by mimicking the action of antithrombin, a natural anticoagulant protein that inhibits Factor Xa. However, unlike antithrombin, which also inhibits thrombin, Fondaparinux selectively targets Factor Xa. This selective inhibition of Factor Xa prevents the formation of thrombin and subsequent clot formation. The mechanism of action of Fondaparinux is distinct from other anticoagulants, such as heparin and warfarin, which have a broader mechanism of action.

**Why Each Wrong Option is Incorrect**
**Option A:** Factor Xa inhibition and Thrombin inhibition - This option is incorrect because Fondaparinux selectively inhibits Factor Xa, but does not inhibit thrombin. This is in contrast to other anticoagulants, such as heparin, which inhibit both Factor Xa and thrombin.

**Option C:** Antithrombin inhibitor - This option is incorrect because Fondaparinux is not an inhibitor of antithrombin. Instead, Fondaparinux mimics the action of antithrombin to selectively inhibit Factor Xa.

**Option D:** Thrombin inhibition - This option is incorrect because Fondaparinux does not inhibit thrombin. Its mechanism of action is selective inhibition of Factor Xa, which indirectly reduces thrombin formation.

**Clinical Pearl / High-Yield Fact**
Fondaparinux is a high-risk medication for bleeding, particularly in patients with a history of renal disease or those taking other anticoagulants. Close monitoring of renal function and platelet count is essential when using Fondaparinux.

**✓ Correct Answer: B. Factor Xa inhibition**