PrepTest 136, Section 2, Question 5
Bacteria that benefit human beings when they are(85 more words not shown)
Which one of the following,(8 more words not shown)
- AStomach ulcers caused(12 more words not shown)
- BCases of tuberculosis(10 more words not shown)
- CPeople who harbor(8 more words not shown)
- DThere are more(12 more words not shown)
- EThere are more(12 more words not shown)
Reading the argument
Ulcers sound bad. But could the same bacterium also do something good? That's the scientists' reason for considering H. pylori a commensal: besides the ulcer risk, it allegedly strengthens the immune response. The passage defines commensals by a benefit to the people carrying them. A harmful effect doesn't, by itself, answer the question of whether a benefit is also present.
The author objects that another bacterium makes about the same percentage of its carriers sick, yet nobody calls that bacterium a commensal. But what benefit does that second bacterium provide? Matching the illness rates compares one side of the story and ignores the reason the scientists gave for their classification.
What we’re looking for
To weaken this comparison, I'd want to learn that M. tuberculosis provides no benefit at all. Then we could explain the different classifications without denying that both bacteria sometimes cause disease. One has an alleged benefit; the other doesn't.
Working the answers
- A
Being able to treat the illnesses doesn't tell us what benefit, if any, comes from harboring either bacterium. Both diseases could respond to antibiotics while only one of the bacteria also helped the person carrying it. This treatment similarity doesn't undermine the author's comparison in the way we need.
- B
A difference in duration could make us wonder about the relative harms, but longer doesn't automatically mean more serious. A brief illness could be more damaging than a longer one. More importantly, B doesn't address the alleged benefit that led scientists to call H. pylori a commensal. C directly identifies the missing difference in that comparison.
- CCorrect
M. tuberculosis provides no benefit to its carriers. That explains why nobody calls it a commensal under the definition we're given. H. pylori, by contrast, allegedly strengthens the immune response. The author can't dismiss that claimed benefit merely by pointing to similar illness rates. C doesn't prove the immune benefit real; it undermines the analogy used to reject the scientists' view.
- D
The number of carriers doesn't determine whether a bacterium benefits a person who carries it. A bacterium could be extremely common and provide no benefit, or uncommon and provide a benefit. More carriers of M. tuberculosis doesn't resolve the missing comparison of effects.
- E
This changes the population comparison in the other direction, but the problem is the same. More people carrying H. pylori doesn't establish whether either bacterium benefits its carriers. We need to compare what the bacteria do to people, not just how many people have them.
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