Definitely random.. one of the problems with subject based is, if you have no idea what the answer is, but you know what subject youre in, it sometimes gives away the answer b/c the others are from other organ systems. So that'll give you a false sense of what you know.
P.S. Is it me or does UWorld just do an awful job of categorizing their questions anyway. When I want to annotate something, on the bottom itll categorize a question say as 'endo/repro' and its clearly a biochem question or something like that.