In a cross-section of the calf, which letter represents the Interosseous membrane?

The choice indicating "Interosseous septum" is correct because the interosseous membrane is a fibrous sheet that connects the tibia and the fibula along their lengths, playing a crucial role in stabilizing these two bones while allowing for mobility of the ankle and foot. The interosseous membrane serves as an attachment site for various muscles and facilitates the distribution of forces between the tibia and fibula.

In the context of the anatomical layout, the interosseous membrane generally lies between the tibia and fibula, making the interosseous septum a more accurate representation of this structure than the other options, which refer to bones or veins. The tibia and fibula are the two primary long bones of the lower leg, whereas the anterior tibial vein is a major vein allowing blood to return from the lower leg. None of these structures reflect the fibrous nature or function of the interosseous membrane.