Which of the following veins is NOT part of the azygous system?

The femoral vein is not part of the azygous system because the azygous system specifically refers to a network of veins located in the thorax that drain the posterior aspect of the thoracic wall and some structures in the thorax, including the intercostal veins. The three principal veins in this system are the azygous vein, the hemiazygous vein, and the accessory hemiazygous vein.

The femoral vein, on the other hand, is located in the lower extremities and is responsible for draining blood from the thigh into the external iliac vein. It has no connection to the structures or functions of the azygous vein system involved in the drainage of the thoracic region. Understanding this anatomical delineation helps clarify the reason why the femoral vein is excluded from the azygous system.