**Core Concept**
A contusion is a type of bruise caused by blood leakage from broken capillaries due to trauma, whereas postmortem hypostasis, also known as livor mortis, is the discoloration of the skin due to the pooling of blood in the dependent parts of the body after death. The key difference lies in the mechanism of blood accumulation and the timing of its occurrence.

**Why the Correct Answer is Right**
To differentiate between a contusion and postmortem hypostasis, one must consider the underlying pathophysiology. Contusions occur due to the disruption of capillaries, leading to the leakage of blood into the surrounding tissues. This process is typically accompanied by the presence of extravasated red blood cells and the breakdown of hemoglobin, resulting in the characteristic bluish discoloration. On the other hand, postmortem hypostasis is caused by the gravitational settling of blood in the dependent areas of the body after death, resulting in a more uniform and symmetrical discoloration. The key distinguishing feature is the presence of a well-defined margin between the affected and unaffected areas in contusions, whereas postmortem hypostasis tends to have a more diffuse and irregular margin.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not address the fundamental difference between contusions and postmortem hypostasis. While the presence of red blood cells may be a feature of both conditions, it is not a reliable method for differentiation.

**Option B:** This option is incorrect because it refers to a feature that is not specific to contusions. The presence of blood in the tissues can be seen in various conditions, including postmortem hypostasis.

**Option C:** This option is incorrect because it refers to a feature that is not unique to postmortem hypostasis. The discoloration of the skin can occur in various conditions, including contusions and other forms of trauma.

**Option D:** This option is incorrect because it does not provide a clear method for differentiation. While the timing of the occurrence may be a factor, it is not a reliable method for distinguishing between contusions and postmortem hypostasis.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that the margin between the affected and unaffected areas in contusions tends to be well-defined, whereas postmortem hypostasis has a more diffuse and irregular margin. This can be a useful clinical clue for differentiating between these two conditions.

**Correct Answer: C. Checking the margin between the affected and unaffected areas can help differentiate between a contusion and postmortem hypostasis.**