When I received an offer from ZS, this is how the interview worked:
The analytical case consisted of the interviewer describing a client situation and then providing many graphs and charts that needed interpretation in order to be able to answer questions such as, "what is the problem?" "what should they do?" "is this solution working?"
The structured case was more traditional (interviewer-led) and required laying using a framework to generate and test hypotheses to use to solve the case, etc. All the basic tools and skills discussed throughout this site. Will likely be pharma-specific and will not require you to be familiar with the industry.
Hope this helps, happy to discuss more by DM, and good luck - it's a good firm and pays well!