## **Core Concept**
Sodium fluoride is used as a preservative in certain types of samples for laboratory testing. It works by inhibiting glycolysis, thereby preventing the breakdown of glucose in the sample. This is particularly important in tests where accurate glucose levels need to be measured.

## **Why the Correct Answer is Right**
The correct answer, **Blood for glucose estimation**, is right because sodium fluoride is used to preserve blood samples for glucose testing. It inhibits the enzyme enolase, which is crucial for glycolysis. By preventing glycolysis, sodium fluoride ensures that the glucose levels in the blood sample remain stable and accurate until the test is performed.

## **Why Each Wrong Option is Incorrect**
- **Option A: Urine for culture**: Sodium fluoride is not typically used for preserving urine for culture. Urine culture preservatives are usually not needed as urine is a sterile environment, but if required, other types of preservatives might be used.
- **Option B: Blood for cross-matching**: Blood for cross-matching is preserved using anticoagulants like citrate or heparin, not sodium fluoride. Sodium fluoride does not prevent clotting.
- **Option C: Tissue for histopathology**: Tissue for histopathology is usually preserved with formalin, not sodium fluoride. Formalin fixation is essential for maintaining tissue architecture.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that when blood glucose levels need to be accurately measured, samples are often preserved with sodium fluoride to prevent glycolysis. This is particularly important in clinical settings where glucose levels can impact diagnosis and treatment decisions.

## **Correct Answer:** D. Blood for glucose estimation.