Immune Thrombocytopenic Purpura (ITP): Given the symptoms of spontaneous bruising, joint pain, and leg swelling, ITP is a strong consideration. It is an autoimmune disease characterized by low platelet count, which could explain the bruising. The joint pain and leg swelling could be related to the underlying autoimmune process or a secondary effect of the thrombocytopenia.

Rheumatoid Arthritis (RA): This condition can cause joint pain and swelling, and while it less commonly causes bruising, it's possible, especially if there's an associated hematologic abnormality.
Systemic Lupus Erythematosus (SLE): SLE is a systemic autoimmune disease that can present with a wide range of symptoms, including joint pain, swelling, and bruising due to its effects on various body systems, including the hematologic system.
Hematologic Malignancies (e.g., Leukemia, Lymphoma): These conditions can cause a variety of symptoms, including bruising, joint pain, and swelling, due to bone marrow infiltration and suppression of normal hematopoiesis.

Disseminated Intravascular Coagulation (DIC): A condition characterized by both widespread clotting and bleeding in the vascular system, which can lead to bruising, organ dysfunction, and potentially life-threatening complications if not promptly recognized and treated.
Sepsis: Although less directly related to the presenting symptoms, sepsis can cause a systemic inflammatory response that might result in similar symptoms, and it is crucial to identify and treat promptly due to its high mortality rate.
Thrombotic Thrombocytopenic Purpura (TTP): A rare disorder of the blood-coagulation system, causing extensive microscopic thrombi to form in the smallest blood vessels throughout the body, which can lead to bruising, neurological symptoms, and renal failure.

Ehlers-Danlos Syndrome: A group of disorders that affect connective tissues supporting the skin, bones, blood vessels, and many other organs and tissues, which could potentially explain the bruising and joint pain due to tissue fragility.
Scurvy: Caused by vitamin C deficiency, leading to connective tissue weakness, which can result in easy bruising and joint pain, although this would be unusual in a previously healthy adult without a specific risk factor for malnutrition.
Amyloidosis: A condition characterized by the deposition of amyloid (a particular type of protein) in various tissues of the body, which can lead to a wide range of symptoms, including bruising and joint pain, depending on the organs involved.