Passage Summary: Scientists think Alzheimer's might be viral because when they gave rats blood from Alzheimer's patients, the rats caught a different neurological disease that we already know is caused by a virus.

Conclusion: Alzheimer's disease might be caused by a virus.

Reasoning: Rats injected with blood from Alzheimer's patients developed a different brain disease that is known to be viral.

Analysis: The argument relies on a significant leap between two different diseases: just because the blood caused Creutzfeldt-Jakob doesn't automatically mean the Alzheimer's itself is viral. To strengthen this, we need to bridge that gap or rule out other explanations for why the rats got sick. Look for an answer choice that suggests a stronger link between the two diseases or confirms that the virus causing the rats' illness actually originated from the Alzheimer's patients' blood. If we can show that these two diseases share a common viral mechanism, the scientist's hypothesis becomes much more plausible.