Acute Peritonitis Is Not Always a Surgical Fix: A Rare Case of Mixed Connective Tissue Disease Presenting as Polyserositis

Mixed connective tissue disease (MCTD) is a complex rheumatologic condition whose diagnosis often presents a challenge to even specialists in the field. Many cases are therefore underrecognized or misdiagnosed due to the heterogeneity of the presentation and manifestations. This report highlights the intricacies of diagnosing a case of MCTD when the presenting symptom is atypical. Herein, we present a case of a young girl who had severe abdominal pain, initially concerning for acute peritonitis from cholecystitis, and was found to have polyserositis affecting the pleural space, pericardium, peritoneum and pelvis secondary to mixed connective tissue disease and adrenal insufficiency.