Plague bacilli, the microorganism that cause bubonic plague, are typically carried from rodents to humans by fleas. Although the bubonic...

GMAT Critical Reasoning : (CR) Questions

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Critical Reasoning

Weaken

MEDIUM

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Plague bacilli, the microorganism that cause bubonic plague, are typically carried from rodents to humans by fleas. Although the bubonic plague epidemic that devastated Europe in the Middle Ages probably originated with certain species of plague-infected rodents from central Asia, the nomadic people of central Asia never contracted plague. Therefore, since they undoubtedly had some contact with infected rodents, it is likely that these central Asian people had psysiological immunity against plague bacilli.

Which of the following, if true, most seriously weakens the argument?

Unlike flea-bearing rodents, which can actually contract bubonic plague, the fleas themselves are merely carriers of disease.

Certain types of plague can pass directly from one human being to another, without fleas or rodent intermediaries.

The plague-infected rodent species that were common in central Asia did not spread to Europe during the Middle Ages.

Plague bacilli were spread from central Asia to Europe by rats inadvertantly transported along with the tradable goods.

The smell of horses, which were a central feature of life among the nomadic people of central Aisa, is repellant to the fleas that live on rodents.

Solution

Passage Analysis:

Text from Passage	Analysis
Plague bacilli, the microorganism that cause bubonic plague, are typically carried from rodents to humans by fleas.	What it says: Plague spreads from rodents to humans through flea bites What it does: Sets up the basic mechanism of how plague transmission works What it is: Background fact about disease transmission
Although the bubonic plague epidemic that devastated Europe in the Middle Ages probably originated with certain species of plague-infected rodents from central Asia, the nomadic people of central Asia never contracted plague.	What it says: Europe's plague came from central Asian rodents, but central Asian people didn't get sick What it does: Presents a puzzling contrast - the source region was unaffected while the destination was devastated What it is: Historical observation Visualization: Central Asia (rodents infected, people healthy) → Europe (massive plague outbreak)
Therefore, since they undoubtedly had some contact with infected rodents, it is likely that these central Asian people had psysiological immunity against plague bacilli.	What it says: Since central Asians must have encountered infected rodents but didn't get sick, they probably had natural immunity What it does: Draws a conclusion that explains the puzzle using the assumption of contact plus the immunity explanation What it is: Author's main conclusion

Argument Flow:

The argument starts by explaining how plague normally spreads, then presents a historical puzzle (plague originated in central Asia but didn't affect the people there), and finally offers immunity as the explanation for this puzzle.

Main Conclusion:

Central Asian nomadic people likely had natural immunity to plague bacilli.

Logical Structure:

The argument assumes that if central Asian people had contact with infected rodents but didn't get plague, then immunity must be the explanation. The logic is: Contact + No Disease = Immunity. However, this overlooks other possible explanations for why they didn't get sick.

Prethinking:

Question type:

Weaken - We need to find information that reduces our belief in the conclusion that central Asian people had physiological immunity against plague bacilli

Precision of Claims

The conclusion specifically claims 'physiological immunity against plague bacilli' based on the assumption that central Asians 'undoubtedly had some contact with infected rodents' but never contracted plague

Strategy

To weaken this argument, we need to find alternative explanations for why central Asian people didn't get plague that don't require physiological immunity. We can attack either the assumption that they had contact with infected rodents, or show that there were other factors preventing transmission that have nothing to do with immunity

Answer Choices Explained

Unlike flea-bearing rodents, which can actually contract bubonic plague, the fleas themselves are merely carriers of disease.

This tells us about the difference between fleas (carriers) and rodents (can contract plague), but this doesn't explain why central Asian people didn't get plague. The transmission mechanism from rodents through fleas to humans would still work the same way, so this doesn't weaken the immunity conclusion.

Certain types of plague can pass directly from one human being to another, without fleas or rodent intermediaries.

This discusses human-to-human transmission, but the argument is specifically about why central Asians didn't get plague from infected rodents in their region. Alternative transmission methods don't explain the original puzzle about rodent-to-human transmission in central Asia.

The plague-infected rodent species that were common in central Asia did not spread to Europe during the Middle Ages.

This is irrelevant to the argument. We're told the plague probably originated from central Asian rodents, but whether those specific species spread to Europe doesn't affect why central Asian people didn't get sick from the infected rodents in their own region.

Plague bacilli were spread from central Asia to Europe by rats inadvertantly transported along with the tradable goods.

This explains how plague got from central Asia to Europe (through rats in trade goods), but again doesn't address why central Asian people themselves didn't contract plague from the infected rodents in their area. The method of spread to Europe is separate from the immunity question.

The smell of horses, which were a central feature of life among the nomadic people of central Aisa, is repellant to the fleas that live on rodents.

This directly weakens the argument by providing an alternative explanation. If horses repel plague-carrying fleas, then nomadic people (whose lives centered around horses) would be protected from flea bites regardless of contact with infected rodents. This means they didn't need physiological immunity - they had environmental protection that prevented transmission, making immunity an unnecessary explanation.

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