Acute SLE-Like Cutaneous Flare Unmasking Mixed Connective Tissue Disease (MCTD) in a 38-Year-Old Female: A Case Report

Purpose: To describe a case in which an acute cutaneous flare mimicking systemic lupus erythematosus (SLE) was ultimately diagnosed as mixed connective tissue disease (MCTD), and to illustrate the diagnostic difficulty posed by overlapping autoimmune syndromes. Methods: Clinical evaluation, complete blood count, coagulation profile, 24-hour urine protein, antinuclear antibody (ANA) testing with titer and pattern, and an extractable nuclear antigen (ENA) antibody panel were performed on a 38-year-old woman presenting with a malar rash, joint pain, mouth ulcers, and a seven-year history of fluctuating symptoms. Results: The patient's presentation, initially attributed to a medication-non-adherence SLE relapse, was instead explained by a positive ANA at a titer of 1:5120 with a speckled pattern and a positive anti-U1-snRNP antibody titer above 1:1600, together with overlapping SLE-, systemic sclerosis-, and myositis-type features that met the 2019 MCTD diagnostic criteria. Treatment with oral prednisolone, hydroxychloroquine, and topical agents produced partial remission. Conclusion: MCTD should be considered whenever SLE-like symptoms do not fully fit a single connective tissue disease, particularly when Raynaud's phenomenon, myositis, or esophageal involvement are present. Anti-U1-snRNP testing is central to reaching the correct diagnosis and guiding treatment.

Keywords: Mixed connective tissue disease; undifferentiated connective tissue disease; ribonucleoprotein, U1 small nuclear; oral ulcer; malar rash; Raynaud's phenomenon; autoimmune.