Unusual Presentation of an Orphan Syndrome Masquerading as Castleman Disease in a Young Adult: Diagnostic Challenges and Clinical Implications: A Case Report.

IgG4-related disease (IgG4-RD) is a rare and diagnostically challenging fibroinflammatory condition characterized by diverse clinical presentations. We present the case of a 33-year-old male with a progressively enlarging right groin swelling over 6 months. A similar episode occurred 4 years prior in Nepal, with an inconclusive biopsy. Initial investigations pointed to chronic subcutaneous infectious granulomatous disease with lymphadenopathy. Following multiple missed appointments, the patient underwent excision of a soft tissue mass and lymph nodes. Histopathology revealed features consistent with IgG4-RD, including follicular hyperplasia and a Castleman disease-like pattern, negative for malignancy. This case underscores the diagnostic challenges of IgG4-RD and the importance of considering it in the differential diagnosis of chronic lymphadenopathy. Timely follow-up, histological evaluation, and awareness of IgG4-RD diagnostic criteria are crucial for proper management and avoiding delayed treatment.