What is your diagnosis? (Congo red is negative as is routine immunofluorescence)
The PAS stain shows glomeruli with significant mesangial and rare capillary loop expansion by a PAS pale, homogenous material while the electron micrograph shows subendothelial, lucent glomerular basement membrane expansion with numerous, embedded, stacked and frayed and slightly curvilinear bundles of electron dense collagen. These findings are consistent with collagenofibrotic glomerulopathy. Of note, this electron micrograph shows the typical appearance of bundles of type III collagen on routine electron microscopic processing, but should tannic acid-lead or phosphotungstic acid preparations be used then superior detail with extensive curvilinear morphologic may be seen (these are the typical images shown in textbooks).
The differential diagnosis of the light microscopy findings is broad and includes amyloidosis, fibrillary glomerulopathy, diabetic fibrillosis and fibronectin glomerulopathy among others. However, the absence of Congo red positivity excludes amyloidosis while the negative immunofluorescence excludes fibrillary glomerulopathy. Fibronectin glomerulopathy shows a different electron microscopic appearance with the absence of collagen fibrils and diabetic fibrillosis typically shows a different electron microscopic appearance and absence of involvement of the glomerular basement membranes.
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