Differential Diagnosis
- Single most likely diagnosis
- Penicillamine-induced membranous nephropathy: This is the most likely diagnosis given the patient's history of rheumatoid arthritis treated with penicillamine and the presence of diffuse fine granular deposits of IgG and C3 in the glomerular basement membrane on renal biopsy. Penicillamine is known to cause membranous nephropathy as a side effect.
- Other Likely diagnoses
- Rheumatoid arthritis-associated membranous nephropathy: The patient's underlying rheumatoid arthritis could also be a cause of membranous nephropathy, independent of the penicillamine treatment. However, the temporal relationship with penicillamine use makes this less likely.
- Lupus nephritis: Although less likely given the specific findings, lupus nephritis can present with various immune complex deposition patterns, including membranous changes. The absence of other systemic lupus erythematosus (SLE) symptoms or serologies makes this less probable.
- Do Not Miss (ddxs that may not be likely, but would be deadly if missed.)
- Goodpasture syndrome: This condition, characterized by anti-GBM antibodies, can present with rapidly progressive glomerulonephritis and should be considered in any case of renal disease with immune deposits, despite the different typical immunofluorescence pattern (linear rather than granular).
- ANCA-associated vasculitis: Given the patient's history of rheumatoid arthritis and the development of renal disease, ANCA-associated vasculitis (e.g., granulomatosis with polyangiitis or microscopic polyangiitis) should be considered, especially if there are systemic symptoms or other organ involvement.
- Rare diagnoses
- Idiopathic membranous nephropathy: While this is a common cause of nephrotic syndrome in adults, the context of penicillamine use and rheumatoid arthritis makes it less likely in this case.
- Other drug-induced nephropathies: Other medications can cause immune complex-mediated glomerulonephritis, but penicillamine is a well-documented cause, making alternatives less likely without additional exposure history.