4192. Its not the subjects themselves, more that Doctor Who hasn't usually been as good when it delves into this sort of area. There are exceptions, so we'll see
Its called Extremis (not related to the Iron Man storyline), I suspect that's more to do with the Latin
"In Extremis", which can mean in a difficult situation, or at the point of death. Given that the actor playing this Doctor is going to be leaving either in this series or at Christmas, and there have been numerous foreboding hints, I suspect that both are probably appropriate!