**Core Concept**
Varicose veins are a type of venous insufficiency resulting from incompetent venous valves and increased venous pressure. Injection sclerotherapy is a minimally invasive procedure used to treat varicose veins by inducing thrombosis and subsequent fibrosis.

**Why the Correct Answer is Right**
The correct answer for injection sclerotherapy for varicose veins is **sodium tetradecyl sulfate (STS)**. STS works by denaturing the protein structure of the endothelial cells lining the varicose vein, leading to thrombosis and eventual fibrosis. This causes the vein to become scarred and closes off, eliminating the varicose appearance.

**Why Each Wrong Option is Incorrect**
* **Option A:** **Sodium morrhuate** is used in sclerotherapy but is more commonly used for spider veins and smaller varicose veins. It's not the preferred choice for larger varicose veins.
* **Option B:** **Polidocanol** is a sclerosing agent used in foam sclerotherapy for varicose veins, but it's not typically used for injection sclerotherapy.
* **Option C:** **Ethanolamine oleate** is a sclerosing agent used in sclerotherapy, but it's not the most commonly used agent for varicose veins.

**Clinical Pearl / High-Yield Fact**
Sodium tetradecyl sulfate is the most commonly used sclerosing agent for injection sclerotherapy of varicose veins due to its efficacy and safety profile. It's essential to note that the choice of sclerosing agent may vary depending on the size and location of the varicose vein.

**Correct Answer:** D. Sodium tetradecyl sulfate