In rapidly progressive glomerulonephritis (RPGN), which histologic finding on light microscopy is considered the defining feature?

Explanation

## Defining Histologic Feature of RPGN **Key Point:** **Crescent formation** is the pathognomonic histologic hallmark of rapidly progressive glomerulonephritis (RPGN). Crescents are composed of proliferating parietal epithelial cells, fibrin, and inflammatory cells that compress and damage the glomerular tuft. ### Definition and Significance A crescent is defined as the accumulation of cells and fibrin in the Bowman space, typically involving >50% of glomeruli in RPGN. The presence of crescents indicates severe glomerular injury and is associated with rapid loss of renal function (days to weeks). ### Classification of RPGN by Immunofluorescence | Type | Immunofluorescence | Cause | Example | |------|-------------------|-------|----------| | **Type I (Anti-GBM)** | Linear IgG | Antibodies to GBM | Goodpasture syndrome | | **Type II (Immune Complex)** | Granular IgG, C3 | Immune complexes | Post-infectious RPGN, lupus | | **Type III (Pauci-immune)** | Negative or minimal | ANCA antibodies | GPA, MPA, EGPA | **High-Yield:** All three types of RPGN share the same light microscopy finding: **crescentic glomerulonephritis**. The immunofluorescence pattern distinguishes the underlying mechanism. ### Clinical Features of RPGN - Rapid decline in GFR (days to weeks) - Hematuria and proteinuria - Hypertension and edema - Systemic symptoms (fever, arthralgia, hemoptysis in pulmonary involvement) - Requires urgent kidney biopsy and immunosuppressive therapy **Clinical Pearl:** The presence of crescents does NOT indicate the etiology — you must use immunofluorescence and clinical context to determine whether it is anti-GBM, immune complex, or ANCA-associated disease.