A renal biopsy specimen stained with PAS stain shows thickening and splitting of the glomerular basement membrane (GBM) with a characteristic 'tram-track' appearance on electron microscopy. Which ultrastructural finding in the GBM is described by this appearance?

• A Fusion of foot processes of podocytes with the GBM
• B Interposition of mesangial cell processes between the endothelium and the GBM, with new GBM formation creating a double contour ✓
• C Subepithelial electron-dense deposits causing GBM thickening
• D Loss of the lamina rara externa with mesangial expansion

Explanation

The 'tram-track' or 'double contour' appearance of the GBM is characteristic of membranoproliferative glomerulonephritis (MPGN). On electron microscopy, mesangial cell processes extend into the subendothelial space and interpose between the endothelial cell and the original GBM. This stimulates new GBM material deposition, creating a double (or even multilayered) GBM structure — the two layers appear as parallel tracks. MPGN type I shows subendothelial deposits; type II (dense deposit disease) shows dense deposits within the lamina densa. Podocyte foot process fusion is seen in minimal change disease and nephrotic syndrome.

Reference: BD Chaurasia's Human Anatomy, 8th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.