Final answer:
The changing colors of a bruise from blue-black to greenish brown to yellow are due to the breakdown of hemoglobin in the blood, resulting in the formation of biliverdin, bilirubin, and urobilin, respectively, as part of the body's natural healing process.
Step-by-step explanation:
The skin discoloration due to a large hemorrhagic area that changes color from blue-black to greenish brown to yellow is indicative of a bruise or contusion. The changing colors result from the breakdown of hemoglobin in the blood, which is part of the body's natural healing process. Initially, hemoglobin in the blood is converted to biliverdin, which imparts a blue-black color to a fresh bruise. Over time, biliverdin breaks down into bilirubin, which has a greenish-brown color, and eventually into urobilin, which has a yellow color. These breakdown products are eventually reabsorbed by the body, causing the bruise to fade over time. Such skin discoloration can be common with physical injuries but can also occur in various medical conditions where bleeding and bruising happen more easily. It's important to differentiate this physiological process from other skin discoloration due to systemic diseases such as liver disease or blood oxygenation issues, which also affect skin appearance but through different mechanisms and with different clinical implications.