## **Core Concept**
The question pertains to the transportation of blood samples for glucose estimation from a Primary Health Centre (PHC) to a laboratory. The core concept here involves understanding the requirements for transporting blood samples, especially for glucose testing, which is crucial for diagnosing and managing diabetes mellitus.

## **Why the Correct Answer is Right**
The correct answer involves the use of **sodium fluoride** and **sodium citrate** or **EDTA** as additives in the blood collection tube for glucose estimation. Sodium fluoride is a glycolysis inhibitor that prevents the breakdown of glucose by cells in the blood sample, thereby ensuring that the glucose level measured is accurate and reflects the true level at the time of collection. This is particularly important for glucose testing because glycolysis can significantly lower glucose levels in vitro.

## **Why Each Wrong Option is Incorrect**
- **Option A:** This option might suggest a method or medium that does not effectively prevent glycolysis or is not suitable for glucose testing. Without specifics, it's hard to directly refute, but generally, any method not using a glycolysis inhibitor would lead to inaccurate glucose readings due to ongoing glycolysis.
- **Option B:** Similarly, this could propose an alternative that doesn't adequately address the need to prevent glycolysis or ensure the integrity of the glucose sample during transport.
- **Option C:** This might imply another incorrect method or material that doesn't serve the purpose of maintaining the sample's integrity for glucose testing.

## **Clinical Pearl / High-Yield Fact**
A crucial point to remember is that for **glucose testing**, blood samples should be collected in tubes containing **sodium fluoride** (and often **EDTA** or **sodium citrate**) to inhibit glycolysis. This ensures that the glucose levels measured are as close as possible to the levels at the time of blood collection, which is vital for the accurate diagnosis and management of diabetes.

## **Correct Answer:** D. Sodium fluoride.