Focal segmental glomerulosclerosis (FSGS) is defined as a clinical-pathologic syndrome manifesting proteinuria and focal and segmental glomerular sclerosis with foot process effacement. The pathologic approach to the classification of FSGS is complicated by the existence of primary (idiopathic) forms and multiple subcategories with etiologic associations, including human immunodeficiency virus (HIV)-associated nephropathy, heroin nephropathy, familial forms, drug toxicities, and a large group of secondary FSGS mediated by structural-functional adaptations to glomerular hyperfiltration. A number of morphologic variants of primary and secondary focal sclerosis are now recognized, including FSGS not otherwise specified (NOS), perihilar, cellular, tip, and collapsing variants. The defining features of these morphologic variants and of the major subcategories of FSGS are discussed with emphasis on distinguishing light microscopic patterns and clinical-pathologic correlations.
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