## **Core Concept**
Fluoride is used as an anticoagulant and glycolysis inhibitor in blood collection tubes. It works by inhibiting the enzyme responsible for glycolysis, thereby preserving glucose levels in the blood sample.

## **Why the Correct Answer is Right**
The correct answer, **D. Enolase**, is right because fluoride inhibits the enzyme enolase. Enolase is a key enzyme in the glycolytic pathway, catalyzing the conversion of 2-phosphoglycerate to enolpyruvate (or phosphoenolpyruvate). By inhibiting enolase, fluoride effectively stops glycolysis in the blood sample, ensuring that glucose levels remain accurate for testing.

## **Why Each Wrong Option is Incorrect**
- **Option A:** Pyruvate kinase is another enzyme in the glycolytic pathway but is not directly inhibited by fluoride. It catalyzes the transfer of a phosphate group from phosphoenolpyruvate (PEP) to ADP, yielding one molecule of pyruvate and one molecule of ATP.
- **Option B:** Hexokinase is the first committed step of glycolysis, converting glucose into glucose-6-phosphate. It is not directly inhibited by fluoride.
- **Option C:** Phosphofructokinase-1 is a key regulatory enzyme in the glycolytic pathway, converting fructose-6-phosphate into fructose-1,6-bisphosphate. While crucial for glycolysis, it is not the target of fluoride inhibition.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that fluoride is specifically used in tubes for collecting blood samples for glucose testing (often paired with sodium citrate or another anticoagulant), because it inhibits glycolysis and thus helps in obtaining accurate glucose levels.

## **Correct Answer:** . Enolase