The interstitial nucleus, a subregion of the brain' s hypothalamus, is typically smaller for male cats than for female cats  A neurobiologist performed autopsies on male cats who died from disease X, a disease affecting no more than   05 percent of male cats, and found that these male cats had interstitial nuclei that were as large as those generally found in female cats
Thus, the size of the interstitial nucleus determines whether or not male cats can contract disease X
The hypothalamus is known not to be causally linked to disease Y, and disease X is a subtype of disease Y.