Mixed connective tissue disease (MCTD) is an autoimmune overlap syndrome with features of SLE, systemic sclerosis (scleroderma), polymyositis, and rheumatoid arthritis, defined by the presence of high-titer anti-U1 RNP antibodies. The pathogenesis involves autoimmune reactivity against the U1 small nuclear ribonucleoprotein (U1 snRNP), a nuclear protein complex involved in RNA splicing. Anti-U1 RNP antibodies drive immune complex-mediated tissue inflammation affecting multiple organ systems. The hallmark clinical features include: Raynaud phenomenon (present in >90% of cases and often the earliest manifestation), swollen 'sausage-like' fingers (puffy hands), polyarthritis, myositis (proximal muscle weakness with elevated CK), esophageal dysmotility, and pulmonary involvement. Pulmonary arterial hypertension (PAH) is the leading cause of death in MCTD and requires regular echocardiographic screening. Unlike SLE, severe renal disease is uncommon in MCTD. The diagnosis requires high-titer anti-U1 RNP antibodies plus clinical features from at least two of the overlap conditions. MCTD often evolves over time, with some patients eventually meeting criteria for a definite connective tissue disease (most commonly SLE or scleroderma).