Final answer:
A dark purplish spot under the nail plate caused by a blood clot is known as a subungual hematoma, often resulting from trauma and bleeding under the nail.
Step-by-step explanation:
The condition in which a blood clot forms under the nail plate causing a dark purplish spot is known as a subungual hematoma. This occurs when trauma to the nail causes bleeding (hemorrhage) and the accumulation of blood under the nail plate. The nail bed, situated beneath the nail plate, typically appears pink due to the presence of capillaries within the dermis.
However, when an injury occurs, the disruption to these tiny blood vessels can lead to petechiae, which manifests as small red or purple spots caused by leaking blood. This hematoma under the nail creates a purplish mark that is often painful. Understanding the health of one's nails, such as observing color changes or testing blood flow with blanch tests, can be an important indicator in identifying various health conditions, including trauma-induced hematomas.