Thanks for the question! (E) isn’t a necessary assumption of the argument, and we can see this by negating it. Let’s say that aluminum is sometimes present in normal brain tissue. So what? We’re told that animals with this behavioral disorder have abnormally high levels of aluminum, and so using this silicon-based compound might help reduce the levels of aluminum to normal levels of aluminum. The truth or falsity of (E) are both consistent with the passage, and so (E) cannot be an assumption on which the argument is based.
Hope this helps! Feel free to ask any other questions that you might have.
