**Core Concept**
Post-mortem staining, also known as livor mortis, refers to the discoloration of the skin in a deceased individual due to the pooling of blood in the lower extremities as a result of gravity. This phenomenon occurs after death and is distinct from bruising, which is a traumatic injury to the skin and underlying tissues.

**Why the Correct Answer is Right**
Post-mortem staining is characterized by its lack of bleeding into the tissue planes, unlike bruising, which involves bleeding into the skin and subcutaneous tissues. The discoloration in post-mortem staining is also typically more uniform and doesn't follow the pattern of a traumatic injury. Additionally, post-mortem staining does not involve the breakdown of red blood cells, which leads to the formation of hemoglobin or bilirubin, whereas bruising does.

**Why Each Wrong Option is Incorrect**
* **Option A:** This answer choice is incorrect because post-mortem staining does not involve the breakdown of red blood cells, whereas bruising does.
* **Option B:** This answer choice is incorrect because post-mortem staining is not typically accompanied by the formation of a palpable mass or swelling, unlike bruising.
* **Option C:** This answer choice is incorrect because post-mortem staining does not follow the pattern of a traumatic injury, unlike bruising.

**Clinical Pearl / High-Yield Fact**
It's essential to note that the color of post-mortem staining can vary depending on the environment and the individual's skin pigmentation, but it is generally more purplish or blue in color due to the pooling of deoxygenated blood.

**Correct Answer:** D