Mice that have been given morphine are very likely to develop blood poisoning because bacteria that normally reside in the...
GMAT Critical Reasoning : (CR) Questions
Mice that have been given morphine are very likely to develop blood poisoning because bacteria that normally reside in the intestine typically respond to morphine by migrating into the bloodstream. However, when mice are given both morphine and the new drug naltrexone, blood poisoning is much less frequent, although it does still occur. These results provide support for researchers' prediction that naltrexone will turn out to be toxic to certain types of bacteria.
Which of the following, if discovered to be true, would most seriously weaken the support for the researchers' prediction?
Passage Analysis:
| Text from Passage | Analysis |
|---|---|
| Mice that have been given morphine are very likely to develop blood poisoning because bacteria that normally reside in the intestine typically respond to morphine by migrating into the bloodstream. |
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| However, when mice are given both morphine and the new drug naltrexone, blood poisoning is much less frequent, although it does still occur. |
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| These results provide support for researchers' prediction that naltrexone will turn out to be toxic to certain types of bacteria. |
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Argument Flow:
The argument starts with a problem (morphine causes blood poisoning through bacterial migration), then shows how a new drug partially solves this problem, and finally concludes that the drug works by being toxic to bacteria.
Main Conclusion:
Naltrexone will turn out to be toxic to certain types of bacteria.
Logical Structure:
The evidence (naltrexone reduces blood poisoning when combined with morphine) supports the conclusion through the assumption that if fewer bacteria are causing problems, naltrexone must be killing them. This is a causal explanation - we see the effect (less blood poisoning) and infer the cause (bacterial toxicity).
Prethinking:
Question type:
Weaken - we need to find information that would reduce belief in the researchers' conclusion that naltrexone is toxic to bacteria
Precision of Claims
The researchers predict naltrexone will be 'toxic to certain types of bacteria' - this is a specific claim about naltrexone's mechanism of action
Strategy
Find alternative explanations for why naltrexone reduces blood poisoning that don't involve killing bacteria. The key insight is that naltrexone could work by affecting the migration process itself rather than by being toxic to bacteria
After being administered to mice, naltrexone does not pass from the bloodstream into the intestine. This doesn't weaken the researchers' prediction because naltrexone could still be toxic to bacteria that have already migrated into the bloodstream. The researchers' prediction is about naltrexone being toxic to certain bacteria, not specifically about where this toxicity occurs. If anything, this might support the prediction since naltrexone would only encounter bacteria in the bloodstream.
Naltrexone inhibits morphine from triggering the migration of intestinal bacteria into the bloodstream. This seriously weakens the researchers' prediction by providing an alternative explanation for naltrexone's effectiveness. Instead of being toxic to bacteria (as the researchers predict), naltrexone could work by preventing the migration process itself. If bacteria never migrate to the bloodstream in the first place, there would be less blood poisoning without naltrexone needing to kill any bacteria. This completely undermines the assumption that naltrexone must be toxic to bacteria.
Mice that have been given naltrexone but not morphine have no greater risk of developing blood poisoning than do mice that have not been given either substance. This actually supports rather than weakens the researchers' prediction. It shows that naltrexone alone doesn't cause blood poisoning, which is consistent with naltrexone being selectively toxic to harmful bacteria rather than being generally harmful.
The increased risk of blood poisoning is not the only harmful effect on mice of being given morphine. This is irrelevant to the researchers' prediction about naltrexone's toxicity to bacteria. The argument focuses specifically on blood poisoning, and other effects of morphine don't affect whether naltrexone works by being toxic to bacteria.
Conditions other than the presence of intestinal bacteria in the bloodstream can cause blood poisoning in mice. This doesn't weaken the researchers' prediction because the argument already acknowledges that blood poisoning 'does still occur' even with naltrexone treatment. The researchers' prediction is specifically about naltrexone being toxic to certain types of bacteria, and other causes of blood poisoning don't contradict this prediction.