We all know that FSGS can exist in both primary and secondary forms.
In both cases light microscopy identifies partially ("segmental") sclerotic glomeruli in a variable ("focal") distribution in the biopsy. How can you tell the different between primary (e.g., idiopathic) versus secondary (e.g., secondary to obesity, hyperfiltration, etc) forms of FSGS on biopsy? The distinction is quite important since primary FSGS often responds to steroids whereas secondary FSGS does not.
At the light microscopy level, it is often difficult to make a distinction between primary & secondary FSGS; however, sometimes with secondary FSGS one observes larger glomeruli, indicative of the compensatory hyperfiltration which takes place in these patients.
More commonly the distinction can be made at the electron microscopy level: primary FSGS will demonstrate diffuse foot process effacement, whereas secondary FSGS usually does not.