LSAT 147 | Section 1 | Logical Reasoning: Q21

LR Question 21 Explanation

QUESTION TEXT: People with higher-than-average blood levels of a…

QUESTION TYPE: Weaken

CONCLUSION: B vitamins in the diet could lower the risk of getting Alzheimers.

REASONING: People with more than average amounts of homocysteine in their blood are more likely to have Alzheimers. B vitamins can transform homocysteine into substances that aren’t associated with Alzheimers.

ANALYSIS: This question makes a causation/correlation error. The author assumes that homocysteine causes Alzheimers. But, it’s possible that Alzheimers is the cause of homocysteine. In any cause/effect situation, it’s always possible that:

1. Alzheimers causes homocysteine.
2. Homocysteine causes Alzheimers. 
3. A third factor causes both.
4. It’s a coincidence.

You can weaken the argument by pointing out one of the alternate possibilities.

___________

1. So? The point is that homocysteine is associated with Alzheimers. It’s possible a disease has several causes.
    
   For example, suppose the real cause of Alzheimers is lack of sleep. It’s possible that homocysteine causes lack of sleep, but also that blue light can cause lack of sleep.
    
   In that case, homocysteine would be a cause of Alzheimers, but not an exclusive cause.
2. So? This isn’t nice for patients, of course, but we only care whether reducing homocysteine would reduce Alzheimers. Side effects are irrelevant.
3. This is a trap answer. The stimulus mentions increasing B vitamins in the diet. It doesn’t mentioned supplements.
4. This shows that genetics are linked with Alzheimers. But this doesn’t weaken the homocysteine explanation. It’s possible these people are genetically predisposed to high homocysteine levels!
5. CORRECT. This weakens the argument, by showing that it got cause and effect backwards. See the analysis above.