## **Core Concept**
Allen's 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 radial artery harvesting for coronary artery bypass grafting (CABG). The test helps determine if the arterial supply to the hand is adequate through the ulnar artery alone.

## **Why the Correct Answer is Right**
The correct answer, **radial artery**, is the one being tested by Allen's test in the context of CABG. During CABG, the radial artery is often used as a graft. Before harvesting the radial artery, it is crucial to confirm that the ulnar artery can provide sufficient blood flow to the hand to prevent ischemia. Allen's test involves compressing both the radial and ulnar arteries at the wrist, releasing one (usually the ulnar), and observing the hand for return of normal color, indicating adequate collateral circulation.

## **Why Each Wrong Option is Incorrect**
- **Option A:** Incorrect because while the **ulnar artery** is involved in the test, the purpose of Allen's test in the context of CABG is to assess the adequacy of circulation if the **radial artery** is harvested.
- **Option B:** Incorrect as it does not directly relate to the specific arteries tested by Allen's test.
- **Option C:** Incorrect because the **internal mammary artery** is not tested by Allen's test; it's another graft option in CABG but not relevant to this specific test.
- **Option D:** Incorrect because it seems to indicate "none of the above," which is incorrect given that one of the options (radial artery) is indeed correct.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that a **positive Allen's test** indicates that it is safe to harvest the radial artery for grafting because the ulnar artery can provide adequate collateral circulation to the hand. This test is a simple yet critical assessment to prevent hand ischemia post-operatively.

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