## Core Concept
The question tests knowledge on the toxicology of arsenic trioxide, specifically the fatal dose in adults. Arsenic trioxide is a highly toxic compound used in various industrial and medical applications. Understanding its toxicity is crucial for managing poisoning cases.

## Why the Correct Answer is Right
The fatal dose of arsenic trioxide in adults is generally considered to be around 100-200 mg. This dose can cause severe gastrointestinal symptoms, cardiovascular collapse, and multi-organ failure, leading to death. The exact dose can vary depending on factors such as the individual's health status, the route of exposure, and the form of arsenic.

## Why Each Wrong Option is Incorrect
* **Option A:** 1-2 mg - This dose is considered therapeutic for certain medical conditions and is much lower than the toxic dose.
* **Option B:** 10-20 mg - While arsenic is toxic, this dose is still relatively low and might cause symptoms but is not typically fatal.
* **Option D:** 500-1000 mg - Although this dose is toxic, it exceeds the generally accepted fatal dose range for arsenic trioxide.

## Clinical Pearl / High-Yield Fact
A key point to remember is that arsenic poisoning can present with gastrointestinal symptoms such as nausea, vomiting, and diarrhea, followed by multi-organ failure. A high index of suspicion is crucial for early diagnosis and treatment.

## Correct Answer: C. 100-200 mg.