A patient presents with cutaneous vasculitis, glomerulonephritis, and peripheral neuropathy. Which investigation should be performed next to help you diagnose the condition?

Explanation

## Correct Answer: B. ANCA The clinical triad of cutaneous vasculitis, glomerulonephritis, and peripheral neuropathy is pathognomonic for **systemic vasculitis**, most commonly **ANCA-associated vasculitis (AAV)**. The three main AAV subtypes—granulomatosis with polyangiitis (GPA, formerly Wegener's), microscopic polyangiitis (MPA), and eosinophilic granulomatosis with polyangiitis (EGPA, formerly Churg-Strauss)—all present with this multi-system involvement. ANCA (anti-neutrophil cytoplasmic antibody) testing is the gold-standard serological investigation for AAV diagnosis. Two main patterns are recognized: **c-ANCA** (cytoplasmic, associated with anti-PR3 antibodies, seen in GPA) and **p-ANCA** (perinuclear, associated with anti-MPO antibodies, seen in MPA and EGPA). The presence of ANCA positivity, combined with clinical features and tissue biopsy showing necrotizing vasculitis, confirms AAV. In the Indian context, AAV is increasingly recognized as a cause of rapidly progressive glomerulonephritis (RPGN) and systemic disease, particularly in young to middle-aged patients. ANCA testing is now widely available in tertiary centers and is essential for early diagnosis and initiation of immunosuppressive therapy to prevent organ damage. ## Why the other options are wrong **A. RA factor** — RA factor (rheumatoid factor) is associated with rheumatoid arthritis and other autoimmune conditions, but it is not specific for systemic vasculitis. While RA-positive patients may develop vasculitis as a complication, RA factor does not explain the acute presentation of cutaneous vasculitis, glomerulonephritis, and neuropathy. This is an NBE trap using a common autoimmune marker to distract from the vasculitis-specific test. **C. HbsAg** — HbsAg (hepatitis B surface antigen) is relevant for hepatitis B serology and is associated with polyarteritis nodosa (PAN) in endemic regions. However, PAN typically presents with medium-vessel vasculitis affecting coronary, renal, and mesenteric vessels, not the small-vessel pattern (cutaneous vasculitis + glomerulonephritis + neuropathy) described here. HbsAg testing would be secondary, not primary. **D. MIF** — MIF (macrophage migration inhibitory factor) is a cytokine involved in immune regulation but is not a standard diagnostic test for vasculitis. This is a distractor option with no clinical relevance to the diagnosis of ANCA-associated vasculitis. MIF is not part of routine vasculitis workup in Indian clinical practice. ## High-Yield Facts - **ANCA-associated vasculitis (AAV)** presents with the triad of cutaneous vasculitis, glomerulonephritis, and peripheral neuropathy—a small-vessel vasculitis pattern. - **c-ANCA (anti-PR3)** is associated with GPA and typically shows upper respiratory and pulmonary involvement; **p-ANCA (anti-MPO)** is associated with MPA and EGPA. - **ANCA positivity** combined with clinical features and necrotizing vasculitis on biopsy confirms AAV diagnosis; negative ANCA does not exclude AAV (seronegative AAV occurs in ~10% of cases). - **RPGN (rapidly progressive glomerulonephritis)** with ANCA positivity is the hallmark renal presentation in AAV; crescent formation is seen on kidney biopsy. - **Immunosuppressive therapy** (corticosteroids + cyclophosphamide or rituximab) is initiated based on ANCA positivity and clinical severity to prevent irreversible organ damage. ## Mnemonics **AAV Triad** **Skin + Kidney + Nerve** = Think ANCA. Cutaneous vasculitis (palpable purpura), glomerulonephritis (hematuria/proteinuria), peripheral neuropathy (mononeuritis multiplex) = small-vessel vasculitis = ANCA-associated. **ANCA Pattern & Disease** **c-ANCA = GPA** (Granulomatosis); **p-ANCA = MPA/EGPA** (Microscopic/Eosinophilic). c-ANCA is cytoplasmic (anti-PR3), p-ANCA is perinuclear (anti-MPO). ## NBE Trap NBE pairs common autoimmune markers (RA factor, HbsAg) with vasculitis to test whether students recognize that ANCA is the specific serological test for small-vessel vasculitis. The triad of cutaneous + renal + neurological involvement is the discriminating clue that points to AAV, not RA or PAN. ## Clinical Pearl In Indian tertiary centers, ANCA-associated vasculitis is increasingly recognized as a cause of RPGN and systemic disease in young patients. Early ANCA testing and diagnosis allow timely initiation of immunosuppressive therapy, preventing progression to end-stage renal disease and permanent neurological damage—critical in resource-limited settings where dialysis access is limited. _Reference: Harrison Ch. 319 (Vasculitis); Robbins Ch. 11 (Vasculitis); KD Tripathi Ch. 18 (Immunosuppressants in vasculitis)_