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What is Azygos vein - and explain in detail?
Azygos vein is formed by

Subcostal veins
Ascending lumbar veins
Lumbar Azygos veins

It crosses the diaphragm on the right side, by piercing the right crus or the aortic opening.
At the level of T4 vertebra, it arches over the hilum of right lung.
It opens into the superior vena cava.