Which histopathological finding best distinguishes Granulomatosis with Polyangiitis (GPA) from Microscopic Polyangiitis (MPA)?

Question

Accepted Answer

A. Granulomatous inflammation with necrosis. ## Distinguishing GPA from MPA

### Key Histopathological Differences

**Key Point:** The hallmark discriminator between GPA and MPA is the presence of **granulomatous inflammation** in GPA, which is **absent in MPA**.

### Comparative Table

| Feature | GPA | MPA |
| --- | --- | --- |
| **Granulomatous inflammation** | Present (necrotizing granulomas) | Absent |
| **Necrotizing vasculitis** | Present (small & medium vessels) | Present (small vessels only) |
| **Organ involvement** | Upper respiratory tract, lungs, kidneys | Kidneys, lungs (rarely upper RT) |
| **ANCA type** | c-ANCA/PR3 (70–90%) | p-ANCA/MPO (70–80%) |
| **Glomerulonephritis** | Necrotizing with crescents | Necrotizing with crescents |

### Pathological Mechanism

**High-Yield:** GPA is characterized by the **triad**:
1. Necrotizing vasculitis of small and medium vessels
2. **Granulomatous inflammation** (the distinguishing feature)
3. Necrotizing glomerulonephritis

MPA presents only necrotizing vasculitis of small vessels **without granulomas**.

### Clinical Pearl

The presence of **necrotizing granulomas** in lung or upper respiratory tract biopsy in a patient with ANCA-positive vasculitis and glomerulonephritis is pathognomonic for GPA. MPA, by definition, lacks granulomatous inflammation.

**Tip:** When comparing ANCA vasculitides on histology, always ask: "Are there granulomas?" If yes → GPA. If no → MPA or GPA-seronegative variant.

[cite:Robbins 10e Ch 11]

![Vasculitis Syndromes diagram](https://mmcphlazjonnzmdysowq.supabase.co/storage/v1/object/public/blog-images/explanation/2963.webp)

Answer

B. Necrotizing vasculitis of small vessels

Answer

C. ANCA positivity on serology

Answer

D. Glomerulonephritis with crescent formation

Which histopathological finding best distinguishes Granulomatosis with Polyangiitis (GPA) from Microscopic Polyangiitis (MPA)?

Explanation

Distinguishing GPA from MPA

Key Histopathological Differences

Comparative Table

Pathological Mechanism

Clinical Pearl

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Feature	GPA	MPA
Granulomatous inflammation	Present (necrotizing granulomas)	Absent
Necrotizing vasculitis	Present (small & medium vessels)	Present (small vessels only)
Organ involvement	Upper respiratory tract, lungs, kidneys	Kidneys, lungs (rarely upper RT)
ANCA type	c-ANCA/PR3 (70–90%)	p-ANCA/MPO (70–80%)
Glomerulonephritis	Necrotizing with crescents	Necrotizing with crescents