Here (a mixture) and I believe in many other unis, we are provided with a very explicit list of learning objectives at the beginning of each semester. It's up to the students to decide which we feel are sufficiently covered by lectures, which we want to develop through PBL sessions and which we feel confident enough to learn in our own time. To that extent, each group sets their own learning objectives in PBL. Each group will develop it's own ideas in PBL, some will do more therapeutics, some more social medicine, some more physiology, but you get taught how to do PBL, and every case should include pathology, relevant physiology, risk factors and causes of disease, treatment and prognosis, psychosocial considerations. Each to a lesser or greater degree.
As for the depth of knowledge needed to answer these questions, the learning objectives provided by the uni should be a guide, but basically it's up to you to guesstimate how important each is. Everyone here learnt in excrutiating detail the pathways of alcohol metabolism because it was our first PBL, only to be told later in the term the whole point of that case was to teach "process" - and so we'd effectively wasted our time (until we did have to learn it in 2nd year). Mostly, it will be obvious what is and isn't important.
In so far as going off on a tangent - PBL is supervised by a staff member (usually a retired Doc. or a PhD who has sheets upon sheets of info from the uni about what is and isn't expected from each case. If you diverge too far from what is expected, or miss something out, you will be dragged back on track.
Hope that helps.