How can I deal with negative marking in a quiz marking scheme?

I wonder if anyone can help me with a problem I have in developing the 'code' for a marking scheme for an exam in Captivate 8. I would be very grateful for pointers towards a viable solution. At the moment I am just thrashing around.

My background is in C++ and I think part of my problem is in trying to find analogues of functions I would use in that environment. My experience with Captivate is limited to developing fairly simple training courses but now I need to put an existing exam on line, maintaining its relatively complicated marking scheme, which is as follows:

The paper is multiple choice, each question having four statements, one (or at the most two) of which is true. A completely correct answer gets 4 marks so where there are two correct statements each gets 2 marks. So far, so easy.

The difficulty arises from an element of negative marking. If 3 or more statements are selected, the score is 0 marks. If there is a single correct statement and it and an incorrect statement are selected, the score is 2 marks. If there are 2 correct statements and one of them is selected, the score is 2 marks. If an incorrect statement is selected as well, the score is 1 mark.

I had assumed I could use conditional logic and user variables on a slide-by-slide basis to provide a score value and then assign it to the appropriate system quiz variable, but it seems I can't assign values to variables in a quiz slide. That would have been too easy...

So, can anybody advise me on an approach that keeps the advantages of using the quiz framework, including the use of the Review function?

Any help very gratefully received.