The 'tram-track' double contour of the GBM in MPGN type I on silver stain results from:

• A Subepithelial immune complex deposits lifting the GBM
• B Splitting of the lamina densa by intramembranous dense deposits
• C Podocyte proliferation forming crescents compressing capillary loops
• D Mesangial cell interposition between the GBM and endothelium with new matrix deposition ✓

Explanation

In MPGN type I, mesangial cells extend peripherally into glomerular capillary walls (mesangial interposition), inserting themselves between the endothelium and the GBM. New basement membrane material is laid down by these interposing cells, creating the characteristic double contour or 'tram-track' on PAS and silver stains. MPGN type II (dense deposit disease) shows intramembranous electron-dense osmiophilic deposits due to C3 nephritic factor stabilising C3 convertase.

Reference: Robbins & Cotran Pathologic Basis of Disease, 10th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.