**Core Concept**
Trendelenburg's test evaluates the functional integrity of the saphenofemoral junction by assessing whether the great saphenous vein (GSV) can maintain its valve function and prevent reflux when the patient is placed in a supine position with the legs elevated. This test specifically detects **saphenofemoral incompetency**, where the valves at the junction between the saphenous vein and the femoral vein fail to prevent backflow.

**Why the Correct Answer is Right**
In Trendelenburg's test, the patient is placed supine with legs elevated. The test involves compressing the saphenous vein and observing for reflux of blood into the superficial venous system. If reflux occurs, it indicates **saphenofemoral incompetency**, meaning the valves at the junction between the great saphenous vein and the femoral vein are incompetent. This is a key step in diagnosing varicose veins and guiding surgical intervention.

**Why Each Wrong Option is Incorrect**
Option A: Perforator competency is assessed in **perforator tests**, not Trendelenburg’s test. This test does not evaluate perforator function.
Option B: Deep vein patency is assessed via **deep venous patency test** (e.g., venography or compression ultrasound), not Trendelenburg’s test.
Option D: The site of perforators is determined via **perforator mapping or imaging**, not this specific test.

**Clinical Pearl / High-Yield Fact**
Trendelenburg’s test is a **simple, non-invasive screening tool** for saphenofemoral incompetence. A positive result (reflux) indicates the need for surgical intervention such as high ligation and stripping of the saphenous vein.

✓ Correct Answer: C. Saphenofemoral incompetency