Final answer:
A red mark beneath a tourniquet is likely petechiae, small spots caused by leaking blood from damaged vessels. Such signs should be monitored as they can indicate injury or infection.
Step-by-step explanation:
If you notice a red mark under a tourniquet, it is most likely petechiae, which are small red or purple spots on the skin that result from blood leaking out of damaged vessels. This can occur if the tourniquet is applied too tightly, causing injury to underlying blood vessels and allowing blood components like erythrocytes and white blood cells to leak out. It is critical to monitor for increased redness, swelling, and pain after an injury or when a wound has been cleaned and bandaged, as these can be signs of infection or other complications.