The interstitial nucleus, a subregion of the brain's hypothalamus, is typically smaller for male cats than for female...

Naz on December 11, 2014

We are told that normally a male cat has a smaller interstitial nucleus than a female cat. However, when a neurobiologist performed autopsies on male cats who had died of disease X, she found that those male cats who had died of this disease had interstitial nuclei that were just as large as those typically found in female cats. Because of this, the argument concludes that the size of the interstitial nucleus determines, i.e. causes, whether or not a male cat can contract disease X.

So a quick recap of our reasoning: we observe that male cats who die of disease X have just as large of an interstitial nucleus as female cats. From this we conclude that the size of the interstitial nucleus determines whether a male cat can contract disease X.

Answer choice (E) states: "The hypothalamus is known not to be causally linked to disease Y, and disease X is a subtype of disease Y."

So we are told that disease Y is not causally linked to the hypothalamus--the region of the brain that the interstitial nucleus resides. We are also told that disease X is a subtype of disease Y, i.e. disease X is a type of disease Y. For example, Hepatitis B is a subtype of Hepatitis. So it is not that X is dependent upon Y, rather X is a type of Y.

Therefore, since the region of the brain that the interstitial nucleus is found is not causally linked to disease Y, which is the overarching disease that disease X is a subtype of, then the size of the interstitial nucleus would not have a causal affect on disease X. Thus, answer choice (E) weakens the argument.

Hope that clears things up! Please let us know if you have any other questions.