**Core Concept**
Oral Factor Xa inhibitors are a class of anticoagulant medications that work by directly inhibiting Factor Xa, a key enzyme in the coagulation cascade. This class of medications is used to prevent and treat thromboembolic events such as deep vein thrombosis and pulmonary embolism.

**Why the Correct Answer is Right**
Oral Factor Xa inhibitors, such as rivaroxaban, apixaban, and edoxaban, work by directly binding to and inhibiting Factor Xa in the coagulation cascade. This prevents the conversion of prothrombin to thrombin, thereby inhibiting the formation of blood clots. **Factor Xa** is a vitamin K-dependent serine protease that plays a critical role in the coagulation cascade, and its inhibition is a key mechanism of action for these medications.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because Factor Xa inhibitors do not work by inhibiting thrombin directly. Thrombin inhibitors, such as dabigatran, are a different class of anticoagulant medications.
**Option B:** This option is incorrect because oral Factor Xa inhibitors do not work by inhibiting Factor IIa (thrombin). Factor IIa inhibitors, such as dabigatran, are a different class of anticoagulant medications.
**Option C:** This option is incorrect because oral Factor Xa inhibitors do not work by inhibiting vitamin K. Vitamin K antagonists, such as warfarin, are a different class of anticoagulant medications that work by inhibiting the production of vitamin K-dependent clotting factors.

**Clinical Pearl / High-Yield Fact**
Oral Factor Xa inhibitors are a convenient and effective treatment option for patients with atrial fibrillation or deep vein thrombosis, and they have a lower risk of bleeding compared to warfarin. However, they require regular monitoring of renal function and dose adjustment in patients with kidney disease.

**Correct Answer:** D.