Passage Summary: A scientist found that male cats with a rare disease had a specific brain part that was unusually large. Because of this, the scientist decided that having a large brain part is what causes the cats to get the disease in the first place.

Conclusion: The size of a male cat's interstitial nucleus is the factor that determines its susceptibility to disease X.

Reasoning: Male cats who died from disease X were found to have interstitial nuclei significantly larger than those of healthy male cats, matching the size typically found in females.

Analysis: This argument falls into the classic 'correlation equals causation' trap. Just because the large nucleus and the disease appear together doesn't mean the nucleus caused the disease. It's just as possible that disease X causes the nucleus to swell, or that some third factor causes both the disease and the enlargement. To weaken this, look for an answer that suggests the disease causes the size change, rather than the other way around. It's a classic case of the biologist potentially putting the cart before the horse—or the symptom before the cause.