PrepTest 147, Section 1, Question 21
People with higher-than-average blood levels of a normal(63 more words not shown)
Which one of the following,(7 more words not shown)
- AMany Alzheimer's patients(4 more words not shown)
- BThe substances into(13 more words not shown)
- CB vitamins and(18 more words not shown)
- DPeople whose relatives(16 more words not shown)
- EAlzheimer's disease tends(9 more words not shown)
Reading the argument
What if Alzheimer's raises homocysteine levels, rather than higher homocysteine levels causing Alzheimer's?
The observed association doesn't settle that direction. A third factor could also explain why the two occur together. For example, some underlying condition might increase both. The author doesn't explicitly announce that homocysteine causes the disease, but the proposed solution relies on its playing that kind of role: reduce homocysteine, and you reduce the risk of Alzheimer's.
What we’re looking for
That won't follow if the elevated homocysteine is just a consequence or a marker of the disease. For the weakening question, I'd look for a reason removing homocysteine might leave the disease risk unchanged.
Working the answers
- A
The argument doesn't say high homocysteine is required for Alzheimer's. A risk factor can increase the chance of a disease even though other people develop the disease without that factor. Normal levels in many patients therefore don't undermine the proposed risk reduction the way a challenge to the causal direction would.
- B
Other harmful effects could matter in deciding whether to follow this recommendation overall. But the specific conclusion is that the dietary change could reduce the risk of Alzheimer's, not that it has no drawbacks or improves every aspect of health. A different harm can coexist with that particular benefit.
- C
This raises a question about one way of getting the vitamins, but inefficient metabolism doesn't mean the body gets no benefit from them. Also, the recommendation is to include them in the diet; it isn't limited to vitamin-mineral supplements. C is closer to the proposed intervention than some other choices, but it leaves both the effectiveness of dietary intake and the alleged causal connection largely open.
- D
A family association doesn't rule out a role for homocysteine. Whatever explains the higher risk among relatives could coexist with, or even operate through, that substance. The family pattern alone also doesn't prove a particular genetic mechanism. We need something that challenges the inference from the blood-level association to this proposed prevention strategy.
- ECorrect
This gives a reason the association could run in the other direction. If Alzheimer's tends to raise homocysteine, higher levels among people diagnosed with the disease needn't mean homocysteine is causing it. Lowering a consequence of the disease might not reduce the risk of developing the disease itself. E doesn't prove that homocysteine has no causal effect whatsoever, but it seriously weakens the study-based reason for thinking that removing it will prevent Alzheimer's.
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