Answer, Image 23: IMMUNOLOGIC
The methenamine silver stain shows inhomogeneity of capillary loop basement membranes; in some cases, fine, whisker-like "spikes" can be seen on the outer surfaces of the basement membranes. In the Masson trichrome stain, a thin "red line" can be seen around the outer surfaces of several loops, suggesting the presence of epimembranous immune complex deposits. The immunofluorescence stain for IgG shows bright, finely granular staining of capillary loop basement membranes, indicating the presence of capillary loop immune complex deposits (compare with the smooth, linear staining pattern in Image 21C). In the electron micrograph, examination of the outer surfaces of capillary loop basement membranes reveals both the immune complexes (granular, electron-dense material) and the intervening "spikes" of basement membrane material (lighter gray).
Diagnosis: idiopathic membranous glomerulonephritis