**Core Concept**
Post-mortem lividity, also known as livor mortis, is the discoloration of the skin due to the pooling of blood in the dependent areas of the body after death. This occurs due to the cessation of circulation and the gravitational settling of blood in the body's lower extremities.

**Why the Correct Answer is Right**
The correct answer is related to the mechanism of post-mortem lividity. Livor mortis does not occur in areas where there is a significant amount of fat tissue, as the blood pools in the tissues rather than the skin. This is because fat tissue has a higher density than the surrounding skin, preventing the blood from accumulating in the skin. Another factor is the presence of gases in the tissues, which can also prevent lividity. However, the most relevant factor in this context is the density of the tissues.

**Why Each Wrong Option is Incorrect**
**Option A:** This option does not provide a valid reason for the absence of post-mortem lividity. While certain medical conditions may affect the pooling of blood, they are not directly related to the absence of lividity.

**Option B:** This option is incorrect because post-mortem lividity can occur in areas with a significant amount of muscle tissue. While the presence of muscle tissue may affect the appearance of lividity, it does not prevent its occurrence.

**Option C:** This option is incorrect because post-mortem lividity can occur in areas with a significant amount of water content, such as the brain. The absence of lividity in such areas is not related to the water content.

**Clinical Pearl / High-Yield Fact**
It's essential to note that post-mortem lividity can be influenced by various factors, including the position of the body, the presence of gas in the tissues, and the density of the tissues. In cases where lividity is not present, it's crucial to consider these factors to determine the underlying cause.

**Correct Answer:** D.