## Core Concept
Postmortem staining, also known as livor mortis or postmortem lividity, is a phenomenon where the skin takes on a discolored appearance after death due to the pooling of blood in the dependent parts of the body. The color can vary, providing clues about the cause of death.

## Why the Correct Answer is Right
The correct answer, **D.**, corresponds to **carbon monoxide poisoning**, which is known to cause a characteristic cherry-red or red-brown discoloration of the skin due to the formation of carboxyhemoglobin. This is a distinctive postmortem finding that can help in the diagnosis of carbon monoxide poisoning.

## Why Each Wrong Option is Incorrect
- **Option A:** This option is incorrect because it does not correspond to a known cause of red-brown postmortem staining.
- **Option B:** This option is incorrect as it does not specify a condition associated with the described postmortem finding.
- **Option C:** This option is incorrect because it does not accurately represent a cause of red-brown discoloration postmortem.

## Clinical Pearl / High-Yield Fact
A key clinical pearl is that **carbon monoxide poisoning** should be suspected in cases of suspected poisoning, especially in enclosed spaces or areas with potential for combustion. The red-brown discoloration is a critical clue that forensic pathologists look for during autopsy.

## Correct Answer Line
**Correct Answer: D.**