**Core Concept**
Post-mortem staining and bruising are two distinct phenomena that occur in the body after death. Post-mortem staining refers to the discoloration of the skin due to the diffusion of blood and other fluids from the underlying tissues into the surrounding skin, whereas bruising involves the extravasation of blood into the skin and subcutaneous tissues due to trauma.

**Why the Correct Answer is Right**
Post-mortem staining typically occurs in areas of the body that are dependent, meaning they are lower than the level of the heart, such as the lower extremities (Option A). The edges of post-mortem staining are often poorly defined, unlike bruises which have distinct borders (Option B). Additionally, post-mortem staining can occur in areas of the body that are under pressure, such as areas compressed by the weight of the body (Option C). However, post-mortem staining does not involve the extravasation of blood into surrounding tissues, unlike bruising (Option D).

**Why Each Wrong Option is Incorrect**
**Option A:** While post-mortem staining often occurs in dependent areas, this is not a distinguishing feature from bruising, as bruises can also occur in dependent areas.
**Option B:** The poorly defined edges of post-mortem staining are a key distinguishing feature from bruises, which have well-defined borders.
**Option C:** Post-mortem staining can occur in areas under pressure, which is not a characteristic that distinguishes it from bruising.

**Clinical Pearl / High-Yield Fact**
It's essential to note that post-mortem staining can be a significant challenge in forensic pathology, as it can be mistaken for bruises or other forms of trauma. Therefore, it's crucial to carefully examine the body and consider the context in which the staining occurred.

**✓ Correct Answer: D. Extravasation of blood into surrounding tissues occur**