A Rare Presentation of Acute Cholecystitis Mimicking Malignancy in a Young Male With Elevated IgG4.

A rare case of a 26-year-old male presenting with abdominal pain and jaundice was initially suspected to be a gallbladder malignancy based on imaging findings of a gallbladder fossa mass and biliary strictures. Serial imaging, including ultrasound, CT, PET-CT, and endoscopic retrograde cholangiopancreatography (ERCP), revealed a hypermetabolic mass with biliary obstruction but no gallstones. Elevated serum IgG4 levels and a marked clinical response to steroids led to the diagnosis of IgG4-related sclerosing cholangitis (IgG4-SC) and cholecystitis, an uncommon presentation that often mimics malignancy or acute cholecystitis. This case highlights the diagnostic challenges and importance of considering IgG4-related disease (IgG4-RD) in young patients with inflammatory biliary masses and emphasizes multidisciplinary management to avoid unnecessary surgery.