IgG4-Related Disease With Lung and Kidney Involvement: A Case Report.

IgG4-related disease (IgG4-RD) is a rare systemic fibroinflammatory condition that often mimics malignancy, infection, or autoimmune diseases, leading to diagnostic delays. Pulmonary and renal involvement in IgG4-RD are rare but pose significant diagnostic challenges. We describe the case of a man in his early 70s who was admitted due to presumed pneumonia and acute kidney injury (AKI). Despite antibiotic therapy, imaging tests revealed progressive pulmonary consolidations and ground-glass opacities. Laboratory tests showed elevated IgG4, hypocomplementemia, and elevated antinuclear antibody titers. Microbiological and cytological studies of bronchoalveolar lavage fluid were unrevealing. Lung biopsy demonstrated IgG4-rich lymphoplasmacytic infiltrates, confirming IgG4-RD. Months later, worsening kidney function led to the diagnosis of IgG4-related tubulointerstitial nephritis. Initially responsive to corticosteroids, the patient relapsed upon tapering. Rituximab was then introduced, achieving disease remission and sustained stability. This case emphasizes IgG4-RD as a "great masquerader," highlighting the need for careful integration of clinical, serological, and histological findings for accurate diagnosis.