I have a character found dead (drowned in a bathtub, if you want the details), and the autopsy doesn't turn up anything conclusive. I want the underlying cause of death to be determined instead by the tox screen, which comes in a little later, and I need that CoD to be some asymptomatic medical condition that was never diagnosed because, well, it was asymptomatic.
I was thinking something that might present like AVM in the sense that it manifests abruptly, but of course anything that involves a major malformation of an organ would be noticed in the autopsy. So I need some kind of medical condition that shows up suddenly, without any real prior symptoms, and kills--or, more accurately, causes unconsciousness/coma, since I want the immediate cause of death to be drowning. The victim needs to still be technically alive when he submerges.