On writing a review (i.e. summary) for a problematic (published) paper I am trying to write a review on a published paper for Mathematical Reviews. The paper proposes a generalization of K-theory. Its main definition is fundamentally problematic. So the rest of the results are worthless. Therefore, I cannot write a brief summary of their results, because it means that I did not understand the mistake. On the other hand, the abstract of the paper is very concise and misleading, so I cannot recommend it as the review to MR either. Besides, I do not want to spend too much time to explain all the mistakes and errors in the paper (it is not my duty as a reviewer!). Therefore I was thinking to write to Mathematical Reviews and let them know about the situation and deny writing any review for this paper. But before doing that, I was wondering if there is a better solution for this problem?

P.S. For those who are not familiar with Mathematical Reviews, I should add that MR asks mathematicians to write brief reviews on papers (book, etc) that are already published and these reviews are available at mathscinet. So, these reviews are different than referee reports.
Your review should do three things (not necessarily in this order). It should tell the reader what the topic of the paper is, perhaps including the authors' main "theorem". (This presupposes that the paper is clear enough to have a topic; I've reviewed garbage that didn't have a discernible topic, but I gather from your question that this is not the case here.) It should make it clear that you think it's wrong. And it should give enough information about the error to allow people in the same area to understand why you think it's wrong. (Once you've explained the essential error, it's not necessary to list a lot of other errors, unless you think that doing so would help the reader or make it clearer that the paper is wrong.)

It is especially important to indicate accurately just how bad the paper is.  For example, is it nonsense, or is it just wrong, or does it give a possibly correct theorem but with inadequate or erroneous proof?  You can save potential readers of the paper a lot of trouble by making the situation clear.