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. Which of the following statements, if true, most seriously weakens the argument? ['The hypothalamus is known not to be causally linked to disease Y, and disease X is a subtype of disease Y.', 'The interstitial nuclei of female cats who contract disease X are larger than those of female cats who do not contract disease X.', 'Of 1, 000 autopsies on male cats who did not contract disease X, 5 revealed interstitial nuclei larger than those of the average male cat.', 'No female cats have been known to contract disease X, which is a subtype of disease Y.']