The sudden appearance of a purple patch on the arm may be caused by trauma. If the arm appears after trauma, it may cause the rupture and bleeding of the subcutaneous capillaries, which may easily lead to subcutaneous bruising and symptoms of swelling and pain in the soft tissues of the skin. In addition this condition, may be suffering from blood system diseases, such as patients with hemophilia, due to the lack of clotting factors, may cause abnormal blood clotting, resulting in subcutaneous bruising. Also patients with hematologic disorders such as thrombocytopenic purpura, leukemia, aplastic anemia, etc., may also develop this condition. In addition to this, the appearance may also be caused by taking medications, such as aspirin and clopidogrel, which are often taken for anti-platelet aggregation therapy, and patients who may also have abnormal clotting. Also patients who apply warfarin, and anticoagulant drugs such as low molecular heparin, may also develop skin bruising. Therefore, when this occurs, further examination at the hospital is needed to clarify the diagnosis and provide targeted treatment.