## Core Concept
Nux vomica poisoning refers to intoxication caused by the seeds of the *Strychnos nux-vomica* plant, which contains the toxic alkaloids **strychnine** and **brucine**. These substances act as central nervous system stimulants. In cases of poisoning, it's crucial to identify and quantify the toxin.

## Why the Correct Answer is Right
The correct answer involves preserving specific biological samples to detect and confirm the presence of strychnine and brucine. **Hair** and **nails** are useful for toxicological analysis because they can store toxins over a long period, providing a wider window for detection compared to blood or urine. This is particularly relevant for chronic poisoning or when the body needs to be cremated, making immediate sampling of tissues impossible.

## Why Each Wrong Option is Incorrect
- **Option A:** While blood and urine are commonly tested for acute poisonings, they are not typically preserved for long-term toxicological analysis like hair and nails.
- **Option B:** CSF (cerebrospinal fluid) might be analyzed in certain cases of poisoning affecting the CNS, but like blood and urine, it's not ideal for long-term storage or analysis of substances like strychnine.
- **Option C:** This option might seem plausible but does not directly relate to common practices in toxicology for detecting substances like strychnine.

## Clinical Pearl / High-Yield Fact
In cases of suspected poisoning, especially with substances that have a delayed effect or are difficult to detect through conventional means (like blood or urine tests), preserving **hair** and **nails** can be crucial for forensic analysis. This is a high-yield fact because it highlights the importance of proper sample handling in toxicology.

## Correct Answer Line
**Correct Answer: D. Hair and nails.**