**Core Concept**
Fluoride is a chemical reagent used to prevent glycolysis in blood glucose estimation samples, thus preventing false low glucose readings. This is a crucial step in the laboratory analysis of blood glucose levels.

**Why the Correct Answer is Right**
In blood glucose estimation samples, fluoride is used to inhibit the enzyme glyceraldehyde-3-phosphate dehydrogenase, which is involved in the glycolytic pathway. By preventing glycolysis, fluoride ensures that the glucose level measured in the sample is accurate. This is particularly important in clinical settings where accurate blood glucose monitoring is essential for diagnosing and managing conditions such as diabetes mellitus.

**Why Each Wrong Option is Incorrect**
* **Option B:** Fluoride is not used to prevent glycolysis in urine glucose estimation samples. In fact, urine glucose estimation samples may contain preservatives such as toluene or mercuric sulfate to prevent bacterial growth and glucose metabolism.
* **Option C:** Fluoride is used in blood glucose estimation samples, but not in urine glucose estimation samples. Therefore, the correct answer is not "Both 1 & 2".
* **Option D:** Fluoride is indeed used in blood glucose estimation samples, making option D incorrect.

**Clinical Pearl / High-Yield Fact**
To remember the role of fluoride in blood glucose estimation, recall that fluoride is used to "freeze" the glucose level at the time of sampling, preventing it from being consumed by cells through glycolysis.

**✓ Correct Answer: A. Blood glucose estimation samples**