Focal segmental glomerulosclerosis (FSGS) is a glomerular disorder that accounts for about 10% of the cases of nephrotic syndrome. FSGS, which affects children and adults, begins as a focal process, affecting only some glomeruli. In the earliest stage, only some of the juxtamedullary glomeruli show changes. Eventually, some glomeruli in other pas of the coex are affected. In the late stages of the disease, the process may become diffuse, affecting most or all glomeruli. Initially, the process is also segmental, involving some but not all of the lobules within an individual glomerular tuft. The involved area shows sclerosis and may show hyalinosis lesions. Eventually some glomeruli show sclerosis of the entire tuft (global sclerosis). Electron microscopy shows increased mesangial matrix and dense granular mesangial deposits. Immunofluorescence typically shows granular mesangial fluorescence for IgM and C3. Because of the focal nature of FSGS, early cases can be difficult to distinguish from minimal change disease (MCD). Clinically, the nephrotic syndrome of FSGS is more severe than that of minimal change disease and is nonselective. The process is much less responsive to steroids and is much more prone to progress to chronic renal failure. It tends to recur in transplanted kidneys. FSGS can be seen in the setting of AIDS nephropathy and heroin nephropathy. Reference: Robbins & Cotran Pathologic Basis of Disease, 9edition