Hey TB - thanks for the question! This is a tricky one, but I'll do my best to help clear it up.
Since you've already spent some time analyzing and understanding this question (and watching its accompanying video), I'll skip a full stimulus deconstruction and instead get right to the heart of the matter.
To start, let's identify the conclusion, and then we'll focus on the correct answer and its negation.
Conclusion: if a trace of mercury is found in Beethoven's hair it will prove that Beethoven had a venereal
disease (which then could have caused his deafness).
Now, to the answer:
(B): Some people in Beethoven's time did not ingest mercury.
The negation of "some did not" is simply "all/everyone did," making the negation essentially, "Everyone in Beethoven's time ingested mercury."
Now think about what that negated statement would do to the argument. It would make it completely absurd: if everyone in Beethoven's time was ingesting mercury, then finding mercury in Beethoven's hair would tell us nothing about the cause, i.e. him having a venereal disease or not...he's just doing the same thing as everyone else, whether diseased or disease-free. So for this argument to begin to make sense it needs to be the case that mercury in Beethoven's hair was somehow special or unique (or at least not a constant for all people alive then).
Would that prove he had a venereal disease as this author is arguing? Not at all. But that isn't the point. We just need to prevent information that would destroy the argument, and that's precisely what (B) is doing!
So you're absolutely correct about this being causal—VD led to the ingestion of mercury—and a good thing to remember about causality in assumptions is that the cause and the effect NEED to be as connected to one another as possible (totally linked, in other words). Negating (B) shows the "effect" (mercury ingestion) would have been present even in the absence of the "cause" (VD), since surely not everyone
alive in Beethoven's time had a venereal disease, yet all were ingesting mercury. Denying that precise cause-without-effect scenario then is something the argument/author need in order to be even potentially plausible, and that makes (B) correct as a Defender Assumption.
Finally, note that there's also an extended discussion of this question elsewhere in this Forum, found here: lsat/viewtopic.php?t=3873&p=34542
. Give that a read if things are still confusing, as I think it will help further explain it