This glomerulus shows a membranoproliferative glomerulonephritis (MPGN) pattern of injury. To a large extent, the etiologic differential diagnosis depends on the immunofluorescence and ultrastructural findings. This light microscopic pattern of injury may be seen in one of the so-called C3 glomerulopathies, in MPGN with immune complexes (no known clinical cause), or in so-called "secondary" forms of MPGN in patients who have underlying infection, autoimmune disease, or dysproteinemia. Identifying the pattern of injury is only the beginning of the workup in many cases!