Where is this question from? The sloppy grammar and unclear modifiers in the answers have me worried about the quality of the source. ("An evidence", "the conclusion that the argument depends on", "a fact that is against the first one" ...) I would avoid poorly edited CR questions like this - CR is hard enough already!

Hi,

B is the right answer. This is my explanation of the answer.

The first Bold face 'these male cats had interstitial nuclei that were as large as those generally found in female cats' states an observation in the diseased cats. And the author uses this in concluding.

But the second bold face 'the hypothalamus is known not to be causally linked to disease Y, and disease X is a subtype of disease Y.' is another evidence which is stated to prove that the conclusion need not be necessarily true.
That is exactly what Option B is doing. Here 'Consideration' means 'Conclusion'. So first bold face is for the conclusion and the second against the conclusion.

Option D:
Its a classic wrong answer. First boldface is an evidence to support and that's right. But the second bold face is a fact against the conclusion and NOT against the evidence in first bold face. So e careful in recognizing the wordings of the answer.

Hope this Helps!

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INNER PEACE

Arguments have a tendency to follow predictable patterns of organization and are always comprised of a conclusion, premise (or evidence), and assumptions (stated or unstated). This is great news because this means any Bolded Statement question can be divided into those three pieces. I suggest you have a predetermined set of symbols you can assign to each sentence. These questions are like "Function"/"Logical Structure" questions in RC. You don't really have to know WHAT the argument is saying, just be able to identify the moving pieces. Here's the symbols I suggest using:

MC = Main Conclusion (the author's argument or position)

OC = Opposing Conclusion (an argument in opposition to the main conclusion)

F = Fact (basic given information, backstory, premise, etc.)

A = Stated Assumption (think of this as part of the passage that "links" given facts/evidence to stated conclusions)

E (+) MC = Evidence Supporting Main Conclusion (this is what the author cites to support his conclusion)

E (+) OC = Evidence Supporting Opposing Conclusion (this is evidence that is cited in support of the opposing conclusion; it undermines the author's conclusion and can also be expressed as E (-) MC).

If you're confused, you can always ask yourself: is this sentence factual or an opinion?

So for this question I'd break down the passage into these symbols: F, E(+)MC, MC, E(-)MC.

So we're looking for an answer choice that represents evidence for AND against the author's conclusion. That's why the answer is between (A) and (B). Here's where we're really splitting hairs. I would actually argue (A) is slightly closer to what the passage's structure describes, but I've never seen an official GMAT question with two choices this close together. What's the source of this one?

Takeaway: Keep everything in general terms for Bolded Statement questions and always have a prediction, and you'll do just fine with even with these convoluted answer choices!