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.