if you forget a critical element of the structure you should let the interviewer know that it is missing and that you would like to consider it as well. It is not an ideal situation, but it is better than ignore the missing part completely.
The way you communicate it is very important. You want to show you noticed there is an additional area but also keep a confident attitude and not disrupt the analysis you are doing.
Say for example you forgot to consider the ways to enter the market in a market entry structure. Below you can find a bad and a good example to communicate it.
Candidate. Oh, I just noticed I completely forgot to consider the ways to enter the market. As that’s a very important point I would like to detail it before we go on with the case.
Candidate. Now, before we go on, I also want to emphasize something I didn’t mention before, that is, we should also consider the possible ways to enter the market. If it is fine for you, we can complete first the analysis of the attractiveness of the industry that you suggested and then review that part as well.
Hope this helps,