**Core Concept**
The best graft for femoral-popliteal bypass surgery involves selecting a suitable material that can withstand the hemodynamic forces and promote long-term patency in the lower limb. The ideal graft should have good compliance, durability, and resistance to infection.

**Why the Correct Answer is Right**
The great saphenous vein (GSV) is the preferred graft for femoral-popliteal bypass due to its excellent biocompatibility, compliance, and resistance to thrombosis. The GSV has a similar diameter and length to the native artery, reducing the risk of graft kinking and promoting better flow. Additionally, the GSV is rich in endothelial cells, which helps to maintain a healthy intimal layer and prevent intimal hyperplasia. The use of the GSV as a graft is associated with a lower risk of graft failure and improved long-term patency rates compared to synthetic grafts.

**Why Each Wrong Option is Incorrect**
**Option A:** Autologous radial artery grafts are not the first choice for femoral-popliteal bypass due to their smaller diameter and higher risk of spasm, which can compromise graft flow.
**Option B:** Dacron grafts are synthetic and may not match the compliance of the native artery, leading to increased risk of graft failure and intimal hyperplasia.
**Option D:** PTFE (polytetrafluoroethylene) grafts are also synthetic and may not provide the same level of biocompatibility and durability as the GSV.

**Clinical Pearl / High-Yield Fact**
When selecting a graft for femoral-popliteal bypass, it's essential to consider the patient's overall health, the severity of their peripheral artery disease, and the availability of suitable graft material. The use of the GSV as a graft is generally recommended for patients with a good quality of life and a low risk of complications.

**Correct Answer:** C. Great saphenous vein.