PrepTest 143, Section 3, Question 4
The size of the spleen is a good(37 more words not shown)
Which one of the following(8 more words not shown)
- APredators are unable(4 more words not shown)
- BMost birds with(8 more words not shown)
- CPredators can sense(5 more words not shown)
- DSickly birds are(10 more words not shown)
- ESmall spleen size(10 more words not shown)
Reading the argument
Small spleens go with poor health. That doesn't yet tell us which causes which. Maybe sickness makes a bird's spleen smaller. Maybe a small spleen contributes to sickness. At this point we only have an association, so I want to keep those possible explanations separate from what the passage establishes.
Then we get a second comparison: birds killed by predators generally had smaller spleens than birds killed accidentally. Since spleen size is a good indicator of health, that suggests the predators' victims tended to be less healthy.
What we’re looking for
Why might that happen? Perhaps predators can detect illness. Or perhaps sick birds are slower and easier to catch, whether or not a predator recognizes the illness. Both are possibilities; neither has been established. The question asks what is most strongly supported, so look for a conclusion about the pattern without pretending we know its cause.
Working the answers
- A
Being less likely to fall prey to a predator doesn't mean being impossible to kill. The findings describe a general difference between the groups, not a rule that every healthy bird escapes. A turns a tendency into an absolute limit on predators' abilities.
- B
This starts with all birds whose spleens are smaller than average and says most die at predators' hands. The study starts with birds already killed and compares spleen sizes across causes of death. Those are different groups. There could be many small-spleened birds that survive, even if predators' victims generally have smaller spleens than birds killed accidentally. We aren't given the proportion of all small-spleened birds that predators kill.
- C
That is one possible explanation, but it isn't a finding of the research. A predator might simply catch the slower bird without sensing that it is sick. The observed pattern doesn't establish the predator's ability to detect illness.
- DCorrect
Sickly birds are more likely than healthy birds to be killed by predators. The health indicator connects the two pieces of information: smaller spleens generally indicate poorer health, and predators' victims generally had smaller spleens than birds killed accidentally. That supports greater vulnerability among sickly birds without requiring us to explain the mechanism or rule out exceptions.
- E
We still don't know the direction of any causal relationship. Illness could make the spleen smaller rather than the small spleen causing illness. Calling spleen size a good indicator of health tells us it is informative; it doesn't tell us it is a main cause of sickness.
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