**Core Concept**
Sodium fluoride is a non-toxic, non-reactive substance used as an anticoagulant in various laboratory tests, particularly in blood glucose and cholesterol assays. It inhibits glycolysis, thus preventing the decrease in glucose levels that would otherwise occur due to cellular metabolism.

**Why the Correct Answer is Right**
Sodium fluoride acts as an anticoagulant by inhibiting the enzyme enolase, which is involved in the glycolytic pathway. This prevents the breakdown of glucose to pyruvate, thereby maintaining the glucose levels in the blood sample. This is particularly useful in assays where accurate glucose measurement is crucial, such as in the diagnosis and management of diabetes mellitus.

**Why Each Wrong Option is Incorrect**
* **Option A:** EDTA (Ethylene diamine tetra-acetic acid) is a commonly used anticoagulant in blood samples, but it is not sodium fluoride.
* **Option B:** Heparin is another anticoagulant used in laboratory tests, particularly in coagulation studies, but it is not sodium fluoride.
* **Option C:** Oxalate is used as an anticoagulant in certain laboratory tests, but it is not sodium fluoride.

**Clinical Pearl / High-Yield Fact**
When collecting blood samples for laboratory tests, it is essential to use the correct anticoagulant to prevent sample degradation and ensure accurate test results. Sodium fluoride is specifically used in blood glucose and cholesterol assays where accurate measurement of glucose levels is crucial.

**Correct Answer:** C.