**Core Concept**
The brown coloration of a contusion is a result of the breakdown of hemoglobin in red blood cells, which leads to the formation of a compound called hematin. This process occurs due to the release of lysosomal enzymes from damaged cells, which cause the degradation of hemoglobin.

**Why the Correct Answer is Right**
The breakdown of hemoglobin leads to the formation of hematin, which is responsible for the brown coloration of contusions. This process is a result of the enzyme hemoglobinase, which is released from lysosomes in damaged cells. Hemoglobinase catalyzes the breakdown of hemoglobin into biliverdin, which is then converted to bilirubin. Bilirubin is responsible for the yellow coloration of bruises, but in the presence of oxygen, it is converted to biliverdin and then to a brown pigment, resulting in the characteristic brown color of contusions.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because the green color of bruises is due to the presence of methemoglobin, not the breakdown of hemoglobin.
* **Option B:** This option is incorrect because the yellow color of bruises is due to the presence of bilirubin, not the breakdown of hemoglobin.
* **Option C:** This option is incorrect because the blue color of bruises is due to the scattering of light by the small vessels in the skin, not the breakdown of hemoglobin.

**Clinical Pearl / High-Yield Fact**
The color of a contusion can be used to estimate its age. The initial red coloration is due to the presence of oxyhemoglobin, which fades to a blue or purple color as the oxygen is released. The yellow coloration is due to the presence of bilirubin, and the brown coloration is due to the breakdown of hemoglobin.

**Correct Answer:** D.