**Core Concept**
The saphenous opening, also known as the saphenous hiatus, is a small anatomical structure located at the junction of the lesser and greater saphenous veins. It serves as a passage for the lesser saphenous vein to enter the deep venous system.

**Why the Correct Answer is Right**
The saphenous opening is a significant landmark in the lower limb venous anatomy. Located at the back of the leg, near the medial malleolus, it allows the lesser saphenous vein to drain into the popliteal vein. This connection is crucial for the proper venous return of blood from the lower limb.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is not relevant to the saphenous opening, which is not related to the femoral vein's drainage.
* **Option B:** The saphenous opening is not associated with the great saphenous vein, which enters the femoral vein through a different route.
* **Option C:** The saphenous opening is not involved in the formation of the deep venous system; rather, it serves as a passage for the lesser saphenous vein to enter the existing deep venous system.

**Clinical Pearl / High-Yield Fact**
The saphenous opening is a crucial anatomical landmark for surgeons and radiologists performing procedures such as femoral vein cannulation or saphenous vein harvesting for coronary artery bypass grafting.

**Correct Answer:** D. The saphenous opening allows the lesser saphenous vein to drain into the popliteal vein.