## Core Concept
The question tests the knowledge of a specific substance that has a dual role in medicine, acting both as a poison and an antidote. This concept is crucial in pharmacology and toxicology, where certain substances can have opposing effects under different conditions.

## Why the Correct Answer is Right
The correct answer, **D. N-acetylcysteine**, acts as both a poison and an antidote. While N-acetylcysteine is primarily used as a mucolytic agent and as an antidote for acetaminophen (paracetamol) overdose, it can be toxic in high doses. High doses can lead to anaphylactoid reactions and even liver damage. However, its role as an antidote, particularly in acetaminophen overdose, is well-established. It works by replenishing glutathione stores in the liver, thereby helping to detoxify the toxic metabolite of acetaminophen.

## Why Each Wrong Option is Incorrect
- **Option A:** This option is incorrect because it does not correspond to a substance known for acting as both a poison and an antidote.
- **Option B:** Similarly, this option does not match a substance with the described dual properties.
- **Option C:** This option is also incorrect as it does not represent a substance known to act as both a poison and an antidote.

## Clinical Pearl / High-Yield Fact
A key point to remember is that N-acetylcysteine is not only used for acetaminophen overdose but also for its mucolytic properties in respiratory conditions. However, its administration can be associated with side effects, particularly anaphylactoid reactions, which necessitates careful monitoring.

## Correct Answer Line
**Correct Answer: D. N-acetylcysteine**