Trauma: The most common cause of a sudden bruise is trauma or injury to the affected area, which could be due to a fall, bump, or any form of blunt trauma. This is likely because bruises are typically a result of blood vessels being damaged, leading to blood leakage into the surrounding tissues.

Vitamin Deficiencies: Deficiencies in vitamins like vitamin C or K can affect blood vessel integrity and clotting, making bruising more likely.
Medication Side Effects: Certain medications, such as anticoagulants (e.g., warfarin, aspirin), can increase the risk of bruising by interfering with blood clotting.
Aging Skin: As people age, their skin becomes thinner and less resilient, making it more susceptible to bruising.

Bleeding Disorders: Conditions like hemophilia or von Willebrand disease can lead to easy bruising due to problems with blood clotting. Missing these diagnoses could lead to significant morbidity or mortality if not properly managed.
Cancer: Certain types of cancer, such as leukemia, can cause bruising due to bone marrow suppression leading to thrombocytopenia (low platelet count).
Infections: Severe infections, including sepsis, can lead to disseminated intravascular coagulation (DIC), a condition characterized by both widespread clotting and bleeding, which could manifest as bruising.

Ehlers-Danlos Syndrome: A group of disorders that affect connective tissues supporting the skin, bones, blood vessels, and many other organs and tissues. Some types can lead to easy bruising.
Henoch-Schönlein Purpura (HSP): A form of vasculitis that affects blood vessels and is characterized by the deposition of immune complexes containing IgA antibodies. It can cause bruising, among other symptoms.
Amyloidosis: A condition characterized by the deposition of amyloid (a particular type of protein) in various tissues of the body, which can lead to bruising due to the disruption of blood vessel integrity.