## **Core Concept**
The Allen test is a medical test used to assess the arterial blood supply to the hand, specifically to evaluate the patency of the radial and ulnar arteries. It is commonly performed before procedures that may compromise the circulation to the hand, such as arterial cannulation or sampling. The test helps in identifying whether one of the two main arteries supplying the hand is occluded.

## **Why the Correct Answer is Right**
The correct answer, **B. Radial artery**, is right because the Allen test is specifically performed to check the patency of the radial and ulnar arteries, ensuring that the hand will still receive adequate blood supply if one of these arteries is used for a procedure like coronary artery bypass grafting (CABG). The radial artery is commonly used as a graft in CABG procedures.

## **Why Each Wrong Option is Incorrect**
- **Option A:** This option is incorrect because while the ulnar artery is also tested by the Allen test, the question seems to focus on the context of CABG, where the radial artery is more commonly used as a graft.
- **Option C:** and **Option D:** are not relevant to the context of the Allen test and CABG.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that a positive Allen test indicates that both the radial and ulnar arteries are patent, ensuring collateral circulation to the hand. This test is crucial before harvesting the radial artery for CABG to prevent hand ischemia.

## **Correct Answer:** B. Radial artery