**Core Concept**
A contusion is a type of bruise caused by bleeding from damaged blood vessels, whereas postmortem staining is the discoloration of tissues after death due to the pooling of blood. The key to differentiating these two conditions lies in understanding their underlying pathophysiology.

**Why the Correct Answer is Right**
The correct answer involves the observation of the presence or absence of bleeding from the damaged blood vessels. In a contusion, there is bleeding from the damaged vessels, which is visible as fresh blood or hemorrhage. In postmortem staining, the blood has already coagulated and is not actively bleeding. Therefore, the correct answer is based on the clinical observation of fresh bleeding from damaged vessels.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because postmortem staining can also involve the pooling of blood in the tissues, making it difficult to distinguish from a contusion based on blood alone.
**Option B:** This option is incorrect because the presence of blood in a contusion does not necessarily indicate fresh bleeding, as the blood may have had time to coagulate.
**Option C:** This option is incorrect because the presence of red blood cells in a contusion does not necessarily indicate fresh bleeding, as the blood may have had time to break down.

**Clinical Pearl / High-Yield Fact**
To differentiate a contusion from postmortem staining, it is essential to observe the presence or absence of fresh bleeding from damaged blood vessels. A quick and simple way to do this is to look for the presence of a "fresh" or "active" bleed, which is often accompanied by a more vivid red color.

**Correct Answer: C. The presence of fresh bleeding from damaged blood vessels.**