**Core Concept**
Thrombotic pulmonary emboli are a common cause of morbidity and mortality, often resulting from the migration of thrombi from other sites in the body. The formation and propagation of these thrombi are influenced by various factors, including venous stasis, hypercoagulability, and endothelial injury.

**Why the Correct Answer is Right**
The most common site of origin for thrombotic pulmonary emboli is the deep leg veins, particularly the femoral and popliteal veins. This is due to the combination of venous stasis, which occurs when blood flow is slow or stagnant, and endothelial injury, which can be caused by various factors such as trauma, surgery, or malignancy. The deep leg veins are prone to thrombosis due to their relatively narrow diameter and the high pressure within them. Additionally, the deep leg veins have a rich network of valves that can become damaged, leading to stasis and increasing the risk of thrombosis. The formation of a thrombus in the deep leg veins can then break loose and migrate to the pulmonary arteries, where it can cause a pulmonary embolism.

**Why Each Wrong Option is Incorrect**
**Option B:** The lumen of the left ventricle is not a common site of origin for thrombotic pulmonary emboli. While the left ventricle can be affected by conditions such as myocardial infarction or cardiomyopathy, these conditions are not typically associated with the formation of pulmonary emboli.

**Option C:** The lumen of the right ventricle is not a common site of origin for thrombotic pulmonary emboli. While the right ventricle can be affected by conditions such as pulmonary hypertension or right heart failure, these conditions are not typically associated with the formation of pulmonary emboli.

**Option D:** The mesenteric veins are not a common site of origin for thrombotic pulmonary emboli. While the mesenteric veins can be affected by conditions such as mesenteric ischemia or pancreatitis, these conditions are not typically associated with the formation of pulmonary emboli.

**Clinical Pearl / High-Yield Fact**
The deep leg veins are a common site of origin for thrombotic pulmonary emboli due to the combination of venous stasis and endothelial injury. This highlights the importance of early mobilization and the use of anti-coagulant therapy in patients at risk of developing deep vein thrombosis.

**✓ Correct Answer: A. Deep leg veins**