## **Core Concept**
Sodium fluoride is used as an in-vitro preservative in blood samples for glucose analysis. It works by inhibiting glycolysis, the process by which cells, particularly red blood cells, break down glucose.

## **Why the Correct Answer is Right**
The correct answer, **D. Enolase**, is right because sodium fluoride inhibits the enzyme enolase. Enolase is a key enzyme in the glycolytic pathway, specifically catalyzing the conversion of 2-phosphoglycerate to enolpyruvate (or phosphoenolpyruvate). By inhibiting enolase, sodium fluoride effectively stops glycolysis, thereby preserving the glucose levels in the blood sample and providing an accurate measurement.

## **Why Each Wrong Option is Incorrect**
- **Option A:** Hexokinase is an enzyme involved in the first step of glycolysis, converting glucose into glucose-6-phosphate. While crucial for glycolysis, sodium fluoride's primary action is not on hexokinase.
- **Option B:** Phosphoglucomutase is involved in glycogenolysis and the synthesis of glycogen, not directly in glycolysis. Sodium fluoride does not primarily act on this enzyme.
- **Option C:** Pyruvate kinase is another enzyme in the glycolytic pathway, catalyzing the final step of glycolysis. Although important for glycolysis, sodium fluoride's mechanism does not primarily involve inhibiting pyruvate kinase.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that sodium fluoride is often used in combination with another preservative, such as sodium citrate or heparin, in blood collection tubes for glucose testing. This combination helps not only to inhibit glycolysis but also to prevent clotting and ensure the sample is properly anticoagulated.

## **Correct Answer:** D. Enolase