**Core Concept:** Systemic thromboembolism refers to the formation and subsequent migration of blood clots that travel to and occlude distant organs or limbs. This can lead to tissue ischemia and infarction. The primary cause is a disturbance in the balance between clot formation (coagulation) and clot dissolution (fibrinolysis).

**Why the Correct Answer is Right:** The correct answer is "D," Deep vein thrombosis (DVT). DVT is a common cause of systemic thromboembolism because it involves a disruption of blood flow in deep veins, particularly in the lower extremities. This can lead to clot formation, which then can dislodge and travel through the venous circulation to reach the lungs and systemic circulation, resulting in pulmonary embolism (PE) and systemic thromboembolism.

**Why Each Wrong Option is Incorrect:**

A. **Option A:** Pulmonary embolism (PE) is a type of systemic thromboembolism, but the question asks for the most common cause, not the general cause of a specific type.

B. **Option B:** Portal vein thrombosis is the primary cause of mesenteric ischemia and infarction, not systemic thromboembolism.

C. **Option C:** Antiphospholipid syndrome is a clotting disorder, but it is not the most common cause of systemic thromboembolism.

E. **Option E:** Venous stasis is a risk factor for DVT but is not the primary cause of systemic thromboembolism.

**Clinical Pearl:** A thorough clinical examination and appropriate imaging studies can help identify risk factors for DVT (e.g., immobilization, trauma, surgery, malignancy), and promptly managing these risk factors can prevent systemic thromboembolism. Prompt diagnosis and treatment of DVT is crucial to prevent serious complications like PE and systemic thromboembolism.