Passage Summary: A researcher found that everyone with a certain brain disease ate the same tropical fruits, while most healthy people didn't, so she concluded the fruit is the cause.

Conclusion: Consuming specific tropical fruits like soursop and custard apple is the cause of atypical Parkinson's disease.

Reasoning: In a study, every single patient with the disease regularly ate these fruits, while only a small fraction of healthy people did.

Analysis: The researcher has found a strong correlation, but she hasn't proven that the fruit is the culprit. To strengthen this, we want to see evidence that these fruits contain something—like a specific toxin—that is known to affect the brain in ways consistent with Parkinson's. We also want to rule out the possibility that people with the disease simply crave these fruits as a result of their condition. Look for an answer that provides a biological link between the fruit and the disease.