In membranous nephropathy, which electron microscopy finding is pathognomonic and best describes the appearance of immune deposits?

Explanation

## Membranous Nephropathy: Electron Microscopy Findings **Key Point:** The "spike and dome" appearance on electron microscopy is the pathognomonic finding in membranous nephropathy, resulting from subepithelial immune complex deposits with intervening basement membrane projections. ### Electron Microscopy Characteristics | Feature | Description | |---------|-------------| | **Deposit Location** | Subepithelial (on the epithelial side of the basement membrane) | | **Appearance** | "Spike and dome" — spikes are basement membrane projections that extend over the deposits | | **Stage** | Evolves from early (small deposits) to late (large confluent deposits with basement membrane thickening) | | **Podocyte Changes** | Foot process effacement (variable, less pronounced than in MCD) | ### Light Microscopy Findings - **Thickened basement membrane** — appears as a homogeneous, eosinophilic ribbon-like structure - **No cellular proliferation** — glomeruli appear normal or mildly enlarged - **Capillary wall thickening** — best seen with silver stains ("tram-track" appearance) ### Immunofluorescence - **Granular pattern** of IgG and C3 deposits along the capillary wall - Distributed in a diffuse, global pattern **High-Yield:** The "spike and dome" appearance is DIAGNOSTIC of membranous nephropathy and is frequently tested. It results from the immune system's attempt to clear subepithelial deposits while the basement membrane regenerates around them. **Mnemonic:** **SPIKE** = **S**ubepithelial deposits, **P**rojections of basement membrane, **I**mmune complexes, **K**eratinous (eosinophilic) appearance, **E**lectron microscopy finding. ### Etiology - **Primary (idiopathic):** Most common; associated with PLA2R antibodies - **Secondary:** Infections (hepatitis B, syphilis), malignancy, drugs (NSAIDs, gold), autoimmune diseases (SLE, rheumatoid arthritis) **Clinical Pearl:** Membranous nephropathy is the most common cause of nephrotic syndrome in adults. The presence of "spike and dome" lesions on EM confirms the diagnosis and helps differentiate it from other nephrotic glomerulonephritides.