Final answer:
A hematoma is usually caused by an injury or trauma that leads to the collection of blood outside the blood vessels. Small veins, inadequate pressure to the site, and needle penetration through the vein can trigger a hematoma. However, petechiae do not typically lead to the formation of a hematoma.
Step-by-step explanation:
A hematoma is a localized collection of blood outside the blood vessels, usually due to an injury or trauma. It can occur when small blood vessels, such as veins, are damaged and blood leaks out into the surrounding tissue. Inadequate pressure to the site and needle penetration all the way through the vein can both contribute to the formation of a hematoma. However, petechiae, which are small red or purple spots on the skin resulting from blood leaking out of damaged vessels, typically do not trigger a hematoma.