Correct Answer: A. Granulomatosis with polyangiitis (GPA)
Pauci-immune glomerulonephritis is characterized by minimal or absent immune complex deposition on immunofluorescence microscopy, despite significant glomerular injury. Granulomatosis with polyangiitis (GPA), formerly Wegener's granulomatosis, is the prototypical pauci-immune ANCA-associated vasculitis (AAV). GPA is mediated by c-ANCA antibodies against proteinase-3 (PR3), which activate neutrophils and cause necrotizing vasculitis of small and medium vessels. The hallmark triad includes upper respiratory tract involvement (sinusitis, nasal granulomas), lower respiratory tract disease (pulmonary nodules, hemoptysis), and glomerulonephritis. On kidney biopsy, GPA shows necrotizing glomerulonephritis with crescent formation, but immunofluorescence reveals minimal or absent immunoglobulin and complement deposition—hence "pauci-immune." This distinguishes it from immune complex-mediated diseases. In Indian clinical practice, GPA is less common than IgA nephropathy but must be recognized for its systemic manifestations and need for aggressive immunosuppression (corticosteroids + cyclophosphamide or rituximab per KDIGO guidelines). The diagnosis is confirmed by positive c-ANCA/PR3 serology combined with necrotizing glomerulonephritis on biopsy.
Why the other options are wrong
B. SLE nephritis — SLE nephritis is an immune complex-mediated glomerulonephritis with prominent immunofluorescence findings (IgG, IgA, IgM, C3, C1q deposits in a granular pattern). It is NOT pauci-immune. The pathophysiology involves circulating immune complex deposition, not ANCA-mediated neutrophil activation. NBE may pair SLE with glomerulonephritis to trap students unfamiliar with immunofluorescence patterns. C. Anti-GBM glomerulonephritis — Anti-GBM disease (Goodpasture syndrome) is characterized by linear immunofluorescence showing IgG and C3 along the basement membrane. Although it causes necrotizing glomerulonephritis with crescent formation, it is NOT pauci-immune because immunofluorescence is strongly positive. It is antibody-mediated against type IV collagen, not ANCA-mediated. This is a common distractor for students confusing necrotizing patterns. D. IgA nephropathy — IgA nephropathy is the most common primary glomerulonephritis worldwide and in India. It shows prominent IgA deposits on immunofluorescence (granular pattern), making it an immune complex disease, not pauci-immune. Although it can progress to crescentic forms, the immunofluorescence pattern is distinctly positive for IgA. ANCA serology is negative. This is a high-yield distractor in Indian exams.
High-Yield Facts
- Pauci-immune GN = minimal/absent immunofluorescence despite necrotizing glomerulonephritis; hallmark of ANCA-associated vasculitis
- GPA (Wegener's) = c-ANCA/PR3-positive necrotizing vasculitis with upper respiratory, lower respiratory, and renal involvement
- Immunofluorescence pattern distinguishes pauci-immune (GPA) from immune complex (SLE, IgA) and linear (anti-GBM) GN
- KDIGO treatment for GPA: induction with corticosteroids + cyclophosphamide or rituximab; maintenance with azathioprine or rituximab
- Indian epidemiology: IgA nephropathy > FSGS > lupus nephritis > ANCA-GN in primary GN; GPA is rare but must be recognized for systemic features
Mnemonics
PAUCI-IMMUNE = ANCA Pauci-immune GN = ANCA-associated vasculitis (GPA, EGPA, MPA). Remember: minimal immunofluorescence but POSITIVE ANCA serology. Use when you see necrotizing GN + negative immunofluorescence + systemic vasculitis symptoms. GPA Triad Granulomatosis = upper respiratory (sinusitis, nasal granulomas) + lower respiratory (lung nodules) + Glomerulonephritis. All three systems involved = think GPA, not other ANCA-GN.
NBE Trap
NBE often pairs "necrotizing glomerulonephritis with crescent formation" with anti-GBM disease to trap students who don't distinguish between linear (anti-GBM) and pauci-immune (GPA) immunofluorescence patterns. The key discriminator is immunofluorescence: GPA has minimal deposits; anti-GBM has strong linear deposits.
Clinical Pearl
In Indian practice, a patient presenting with sinusitis + hemoptysis + hematuria should raise suspicion for GPA. Perform c-ANCA/PR3 serology and kidney biopsy immediately; pauci-immune pattern on IF with necrotizing GN confirms diagnosis. Early recognition prevents rapid progression to end-stage renal disease.
_Reference: Harrison Ch. 279 (Glomerular Diseases); Robbins Ch. 20 (Kidney); KD Tripathi Ch. 14 (Immunosuppressants in vasculitis)_