PrepTest 147

Incorrect. This answer is trying to get you to think that it's an example of method #2 from our list of common weakeners—presenting a counterexample, or examples of the effect without the cause. However, the stimulus was probabilistic, not definite—it left open the possibility that there could be other causes of Alzheimer's. So this answer doesn't weaken the argument.

Incorrect. The conclusion is only about lowering Alzheimer's risk, not overall health or whether these other substances should be taken, so this answer is out of scope.

Incorrect. The stimulus seems to take this into account by saying that "large amounts" of these vitamins must be taken. It also doesn't say anything about taking them as vitamin-mineral supplements. This answer is out of scope.

Incorrect. This answer can seem like it's identifying an alternative cause—genetics. However, as noted in (A), the stimulus leaves open the possibility that there are multiple causes of Alzheimer's, so this answer doesn't weaken it. This answer would have to establish that genetics would both raise homocysteine levels and raise Alheimer's risk to serve as an alternative cause.

Correct. Argument or Facts:
Argument

Valid or Flawed:
Flawed

Question Type:
Weaken

Stimulus Summary:
High homocysteine levels are correlated with Alzheimer's disease, so converting homocysteine to other substances should reduce Alzheimer's risk.

Answer Anticipation:
Weaken questions frequently feature correlation/causation flaws, so we should be on the lookout for language in the stimulus that raises these concepts. And right off the bat, we get some—someone with a certain substance is "twice as likely" to be diagnosed with Alzheimer's. Language about likelihood is almost always correlative (unless paired with causal language, e.g., "increases the likelihood").

Correlative language in the premises is only half the flaw—there needs to be causal language in the conclusion. And here, there is—something would "reduce[]" the risk of Alzheimer's. So the argument jumps from high homocysteine levels being correlated with Alzheimer's to it being a cause of Alzheimer's (hence lowering levels would lower risk).

Generally, there are three ways that we can weaken an argument with a correlation/causation flaw:

(1) ID an alternative cause (certainly possible here—maybe a disease increases Alzheimer's risk and homocysteine levels—but less likely because the stimulus doesn't state that high homocysteine levels are the only contributing factor to Alzheimer's)

(2) Counterexamples (less likely here since the stimulus is probabilistic—homocysteine doesn't always cause Alzheimer's, just increases the risk of it)

(3) Reversed causality

And, in general, when the third makes sense, it's the correct answer. This trend is pretty strong in medical questions such as this one, as well. So while we should keep our eyes open for an answer that IDs an alternative cause, we should primarily look for an answer that states Alzheimer's can cause elevated levels of homocysteine.

Answer Explanation:
This answer brings up the possibility of the argument concluding the wrong causal relationship, treating a cause as an effect. If Alzheimer's raises homocysteine levels, then the conclusion falls apart, and so this weakens the argument.

Key Takeaway:
When the reverse causal relationship to the one raised in the conclusion makes sense in an argument with a correlation/causation flaw, the correct answer to a Weaken question will generally bring it up. This is especially true when the topic is medical in nature!