**Core Concept**
The use of sodium fluoride (NaF) as a preservative in suspected cases of poisoning is a critical step in forensic analysis, particularly in cases involving the degradation of volatile substances. NaF acts by inhibiting the microbial growth that can cause spoilage and degradation of the evidence.

**Why the Correct Answer is Right**
In suspected cases of alcohol poisoning, NaF is added to the sample as a preservative to prevent the degradation of ethanol. Ethanol is a volatile substance that can evaporate or be metabolized by microorganisms, leading to inaccurate test results. NaF inhibits the growth of microorganisms, thereby preserving the ethanol in the sample for further analysis. This is crucial in forensic medicine, as accurate determination of blood alcohol concentration is essential in cases of suspected driving under the influence or other alcohol-related offenses.

**Why Each Wrong Option is Incorrect**
**Option A:** Cocaine is not typically preserved with NaF, as it is a stable compound that does not degrade readily. Instead, cocaine samples are often preserved with other chemicals, such as sulfuric acid or sodium hydroxide.
**Option C:** Cyanide is a highly toxic substance that can be detected using various analytical techniques, but it is not typically preserved with NaF. Cyanide samples are often preserved using other methods, such as freezing or using a cyanide-specific preservative.

**Clinical Pearl / High-Yield Fact**
In forensic medicine, it's essential to preserve evidence correctly to ensure accurate analysis and prevent contamination or degradation of the sample. NaF is a commonly used preservative for suspected cases of alcohol poisoning, but other substances may require different preservation methods.

**✓ Correct Answer: B. Alcohol. NaF is added in suspected cases of alcohol poisoning to prevent degradation of ethanol.**