The structure indicated is the head of the fibula.

The two bones of the leg are the tibia and the fibula. The fibula is the smaller of the two bones and lies laterally. The head of the fibula is an expansion of the fibula at the proximal end, and is flattened superiorly and medially forming a surface for articulation with the lateral condyle of the tibia.

The head of the fibula provides the point of attachment for several muscles and ligaments, including the biceps femoris and the lateral collateral ligament (fibular collateral ligament).

Just below the head of the fibula, the common peroneal nerve winds around the neck of the fibula – a point of exposure making it susceptible to damage.

Learn more about the anatomy of the fibula in this tutorial.