Great Saphenous Vein
The structure indicated is the great saphenous vein.
The great saphenous vein is a large superficial vein (the longest vein in the human body), which begins from the medial aspect of the dorsal venous arch of the foot. It then passes anteriorly, in front of the medial malleolus and ascends medially up the lower limb to drain into the femoral vein in the femoral triangle.
Learn about the femoral triangle in this anatomy tutorial.
Learn about the veins of the body in this tutorial.