When one thinks about glomerular diseases in Nephrology; its interesting that we always have found some secondary causes for everything. All diseases from Membranous GN, Minimal Change, MPGN and igA Nephropathy have known secondary causes and one has to rule them out first. If no cause is found, we hence call it "idiot"iopathic - since we are idiots about the cause.
If one looks at each disease specifically, IgA Nephropathy probably has the least known secondary causes and MPGN likely has the most known secondary causes. The rest would fall somewhere in between. Perhaps one day- the list of primary idiopathic will be smaller and known causes can be identified.
Of all the glomerular diseases, when one encounter's MPGN, think secondary and go back to the history and see if there are clues to an underlying secondary cause.