I remember, way back, practising just this scenario on an R22 safety course. Once you've practised it once, you know what to do. If you haven't, then in an emergency it's hard to troubleshoot and think clearly, and I can quite see that your first reaction might be to enter autorotation. After that, the average PPL would be so busy looking for somewhere to land, he wouldn't have time and brain capacity for much else.
It's worth remembering that being put on the spot by an instructor almost puts some people's brains into emergency mode...they're liable to answer instinctively and not be sure what to do. But if that's the case, they won't forget the right answer...which could save their lives later on.
It's easy to know all the answers down here; much harder up there.