**Core Concept**
The most commonly used vein for intravenous injection is selected based on its accessibility, size, and ease of cannulation. The ideal vein should have a diameter of at least 2 mm and a length of 2-3 cm to accommodate the cannula without causing undue discomfort or risk of extravasation.

**Why the Correct Answer is Right**
The cephalic vein is the most commonly used vein for intravenous injection due to its superficial location and accessibility, particularly in the forearm. It is a larger vein compared to other superficial veins, making it easier to cannulate. The cephalic vein is also less prone to thrombosis and phlebitis compared to other veins, reducing the risk of complications.

**Why Each Wrong Option is Incorrect**
* **Option A:** The basilic vein is a deeper vein compared to the cephalic vein, making it more difficult to access. It is also more prone to thrombosis and phlebitis.
* **Option B:** The median cubital vein is a smaller vein that is often used for blood sampling, but it is not the most suitable vein for intravenous injection due to its smaller size and higher risk of complications.
* **Option C:** The median cephalic vein is not a commonly used vein for intravenous injection, as it is a smaller and more fragile vein compared to the cephalic vein.

**Clinical Pearl / High-Yield Fact**
When selecting a vein for intravenous injection, it is essential to choose a vein with a diameter of at least 2 mm and a length of 2-3 cm to minimize the risk of complications and ensure smooth cannulation.

**Correct Answer: C. Cephalic vein.**