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What is Focal glomerulonephritis can be seen in all, except- and explain in detail?
Remember following important facts

RPGN is the severe form of acute proliferative (diffuse or focal) glomerulonephritis.
When acute proliferative glomerulonephritis becomes severe, there is a rapid deterioration of renal function and there is the formation of the crescent, it is referred to as rapidly progressive glomerulonephritis.
So, causes of RPGN, focal proliferative glomerulonephritis and diffuse proliferative glomerulonephritis are the same.
For causes see above explanation.
In a milder form, they cause proliferative (focal or diffuse) glomerulonephritis and when it becomes severe it is referred to as RPGN.