Trauma: The most likely cause of purpura on a specific area like the right knee and lower leg is trauma, which could be due to a fall, hit, or any form of injury that causes blood vessels to break and leak into the skin.

Vasculitis: Conditions that cause inflammation of the blood vessels can lead to purpura. This could be due to autoimmune diseases, infections, or allergic reactions.
Thrombocytopenia: A low platelet count can increase the risk of bleeding into the skin, leading to purpura. This could be due to various causes, including medication side effects, bone marrow disorders, or immune system diseases.
Coagulopathy: Disorders that affect the blood's ability to clot, such as hemophilia or liver disease, can also lead to purpura.

Deep Vein Thrombosis (DVT): While less common as a direct cause of purpura, DVT can lead to skin discoloration and should be considered, especially if there's swelling and warmth of the leg.
Infection (e.g., Septicemia, Meningococcemia): Certain infections can cause purpura and are medical emergencies. Meningococcemia, for example, can cause a characteristic rash that may start as purpura.
Disseminated Intravascular Coagulation (DIC): A serious disorder in which the proteins responsible for blood clotting become overactive, leading to widespread clotting and bleeding in various parts of the body.

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's more common in children but can occur in adults.
Amyloidosis: A condition characterized by the deposition of amyloid (a particular type of protein) in various tissues of the body, which can lead to purpura, among other symptoms.
Scurvy: Vitamin C deficiency can lead to weakened blood vessel walls, resulting in purpura. This is rare in developed countries but can occur in individuals with poor nutrition.