I think PhD courses are virtually impossible to fail (i.e sub-B) if you at least put some time into it. Example: one of my best friends, who has NO (zero) math training in undergrad (not even calculus, he's a philosophy major), was able to get As for all his econ courses in a top 10 quant marketing program. Granted he worked his *** off, but just to say that you don't have to worry if you haven't read MWG for a while (or, in my case, never, lol).