Navigating the Unknown: Scleroderma Renal Crisis and a Fatal Cardiac Complication in a Young Patient With MCTD With Coexisting Anti-U3 RNP and Anti-RNA Polymerase III Antibodies.

We report the case of a 30-year-old Somali female with mixed connective tissue disease (MCTD) who developed dermatomyositis and both scleroderma renal crisis (SRC) and fatal cardiac complications. Notably, she tested positive for both anti-U3 RNP and anti-RNA polymerase III antibodies which is a rare serologic combination. After initial treatment with corticosteroids for dermatomyositis features, she experienced rapid clinical deterioration marked by malignant hypertension and thrombotic microangiopathy and was found to have SRC and diffuse myocardial fibrosis. Despite treatment with captopril and hemodialysis, she ultimately died of cardiac arrest. This case highlights the complexity and risks in managing patients with overlapping autoantibodies and underscores the need for close monitoring and cautious use of steroids in such high-risk individuals.