A 35-year-old woman with biopsy-proven SLE is found to have proteinuria on routine urinalysis. Which is the most common pattern of lupus nephritis on renal biopsy?

Explanation

## Most Common Pattern of Lupus Nephritis **Key Point:** Class IV (diffuse proliferative) lupus nephritis is the most common and most severe form, occurring in approximately 40–50% of SLE patients with renal involvement. ### WHO/ISN-RPS Classification of Lupus Nephritis | Class | Pattern | Frequency | Prognosis | Key Features | |-------|---------|-----------|-----------|---------------| | I | Minimal mesangial | <5% | Excellent | Normal on LM, immune deposits on IF | | II | Mesangial proliferative | 10–15% | Good | Mesangial proliferation, <50% glomeruli | | III | Focal proliferative | 20–25% | Good to fair | <50% glomeruli involved, endocapillary or extracapillary proliferation | | **IV** | **Diffuse proliferative** | **40–50%** | **Fair to poor** | **>50% glomeruli, wire-loop lesions, hyaline thrombi** | | V | Membranous | 10–20% | Variable | Thickened GBM, subepithelial deposits ("spike and dome") | | VI | Advanced sclerosing | <5% | Poor | >90% glomeruli sclerosed | **Clinical Pearl:** Class IV is the most aggressive form and most commonly presents with nephrotic or nephritic syndrome. Patients with Class IV have the highest risk of progression to ESRD if untreated. **High-Yield:** - Class IV lupus nephritis is characterized by **wire-loop lesions** and **hyaline thrombi** on light microscopy - Immunofluorescence shows **full house pattern**: IgG, IgA, IgM, C1q, C3 (all positive) - Electron microscopy reveals **subendothelial and subepithelial deposits** ("garland" pattern) **Mnemonic:** **FLIP-IN** for lupus nephritis severity order: - **F**ocal (Class III) - **L**ess common (Class II, V) - **I**nitial (Class I) - **P**roliferative diffuse (Class IV) — most severe - **I**ncreased sclerosis (Class VI) - **N**ormal-appearing (Class I) **Warning:** Do not confuse Class III (focal) with Class IV (diffuse). The key difference is the percentage of glomeruli involved: <50% = focal; >50% = diffuse. Class IV has worse prognosis and requires more aggressive immunosuppression.