The question stem asks us to find the one answer choice that could not be true; in other words, the correct answer must be false. This means we need to eliminate everything that's compatible with the stimulus and choose whichever answer contradicts the stimulus. And of course, the first step to do that is to understand what the stimulus says.

Although the stimulus lays out an argument, for a MBF question we can take every claim for granted, meaning the support structure of the argument doesn't matter. So assuming everything the stimulus claims is true, these are the facts: (1) identical twins have genetically identical brains; (2) it's possible that only one of a pair of identical twins has schizophrenia; (3) when only one twin has schizophrenia, the schizophrenic twin has certain brain areas that are smaller; (4) this difference only happens when one twin has schizophrenia; (5) schizophrenia is caused by brain damage.

For each answer choice, we want to check if it contradicts one of these facts. If it does, then it must be false, which will be our correct answer. If it doesn't, then it could be true and we can eliminate. We should also keep in mind the difference between "could" and "must" be true—the incorrect answers don't need to be guaranteed, they just need to be possible.