**Core Concept**
Postmortem staining, also known as livor mortis, is the discoloration of the skin and underlying tissues due to the pooling of blood in the venous system after death. This phenomenon is a result of the cessation of circulation and the subsequent gravitational settling of blood in the dependent areas of the body.

**Why the Correct Answer is Right**
Livor mortis occurs immediately after death, as the circulatory system stops functioning and blood is no longer circulated. It is more pronounced in the dependent parts of the body, such as the lower back, buttocks, and legs, due to gravity. The margins of livor mortis are typically flat, not raised. Additionally, livor mortis can be observed in internal organs, although it may be more difficult to detect. The discoloration of livor mortis can be maintained even after rigor mortis has set in, as it is a separate process.

**Why Each Wrong Option is Incorrect**
* **Option A:** Livor mortis does not occur immediately after death, but rather after a period of time, typically 2-4 hours, as the blood settles in the dependent areas.
* **Option D:** The margins of livor mortis are typically flat, not raised.

**Clinical Pearl / High-Yield Fact**
It's essential to note that livor mortis can be mistaken for other conditions, such as bruising or ecchymosis, and should be considered in the context of the overall postmortem examination.

**Correct Answer: E. Not found in internal organ**