Lyme disease is caused by a bacterium transmitted to humans by deer ticks. Generally deer ticks pick up the bacterium...

GMAT Critical Reasoning : (CR) Questions

Source: Official Guide

Critical Reasoning

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Lyme disease is caused by a bacterium transmitted to humans by deer ticks. Generally deer ticks pick up the bacterium while in the larval stage from feeding on infected whitefooted mice. However, certain other species on which the larvae feed do not harbor the bacterium. Therefore, if the population of these other species were increased, the number of ticks acquiring the bacterium and hence the number of people contracting Lyme disease—would likely decline.

Which of the following it would be most useful to ascertain in evaluating the argument?

Whether populations of the other species on which deer tick larvae feed are found only in areas also inhabited by white footed mice.

Whether the size of the deer tick population is currently limited by the availability of animals for ticks 's larval stage to feed on

Whether the infected deer tick population could be controlled by increasing the number of animals that prey on white footed mice.

Whether deer ticks that were not infected as larvae can become infected as adults by feeding on deer on which infected deer ticks have fed.

Whether the other species on which deer tick larvae feed harbor any other bacteria that ticks transmits to humans.

Solution

Passage Analysis:

Text from Passage	Analysis
Lyme disease is caused by a bacterium transmitted to humans by deer ticks.	What it says: Deer ticks spread a bacterium that causes Lyme disease in people What it does: Sets up the basic cause-and-effect relationship we need to understand What it is: Author's factual background
Generally deer ticks pick up the bacterium while in the larval stage from feeding on infected whitefooted mice.	What it says: Tick larvae get the bacterium by feeding on infected mice What it does: Explains how ticks become carriers in the first place, building on the transmission chain What it is: Author's explanation of the process Visualization: Mice (80% infected) → Tick larvae feed → Ticks become carriers → Humans get sick
However, certain other species on which the larvae feed do not harbor the bacterium.	What it says: Some animals that tick larvae feed on don't carry the bacterium What it does: Introduces a contrasting scenario that could disrupt the infection chain What it is: Author's contrasting fact Visualization: Clean animals (mice-alternatives, 0% infected) vs. Infected mice (80% infected)
Therefore, if the population of these other species were increased, the number of ticks acquiring the bacterium and hence the number of people contracting Lyme disease—would likely decline.	What it says: More clean animals would mean fewer infected ticks and less human Lyme disease What it does: Draws the logical conclusion from all the previous facts about the transmission chain What it is: Author's main conclusion Visualization: More clean animals (70% of food sources) → Fewer ticks get bacterium (30% vs. 80%) → Less human Lyme disease (decline from 100 to 30 cases)

Argument Flow:

The argument starts with basic facts about Lyme disease transmission, then explains how ticks become infected, introduces the key fact about clean alternative food sources, and concludes that increasing these alternatives would reduce disease rates.

Main Conclusion:

Increasing the population of animal species that don't carry the Lyme bacterium would likely reduce the number of people getting Lyme disease.

Logical Structure:

The author uses a chain of causation: since ticks get the bacterium from infected mice, and some animals don't carry the bacterium, then having more of these clean animals as food sources would break the infection chain and reduce human disease rates.

Prethinking:

Question type:

Evaluate - We need to find information that would help us determine whether the proposed solution (increasing populations of clean animals) would actually work to reduce Lyme disease

Precision of Claims

The argument makes specific claims about causation (bacterium causes disease), transmission mechanism (larvae feeding creates infected ticks), and a quantitative prediction (more clean animals = fewer infected ticks = less disease)

Strategy

For evaluate questions, we need to think about what assumptions the argument relies on and what information would either strengthen or weaken the conclusion when taken to extremes. We should focus on gaps in the logical chain from 'more clean animals' to 'less human Lyme disease'

Answer Choices Explained

Whether populations of the other species on which deer tick larvae feed are found only in areas also inhabited by white footed mice.

This focuses on geographic distribution, but the argument's logic doesn't depend on whether these species coexist in the same areas. Even if they're in different locations, the principle that more non-infected food sources would reduce infection rates could still hold within areas where both exist. This doesn't directly help us evaluate the core assumption about whether increasing non-infected animals would actually change tick feeding patterns.

Whether the size of the deer tick population is currently limited by the availability of animals for ticks 's larval stage to feed on

This directly addresses a critical assumption in the argument. If tick populations aren't limited by food availability, then adding more non-infected animals wouldn't change how many larvae feed on infected mice - they'd just have more total food options. But if ticks are food-limited, then more non-infected animals could effectively crowd out infected mice as food sources. This information is essential for determining whether the proposed solution would actually work.

Whether the infected deer tick population could be controlled by increasing the number of animals that prey on white footed mice.

This suggests an alternative solution (reducing infected mice through predation) rather than evaluating the proposed solution (increasing non-infected alternative food sources). While potentially relevant to Lyme disease control generally, it doesn't help us assess whether the specific strategy mentioned in the argument would be effective.

Whether deer ticks that were not infected as larvae can become infected as adults by feeding on deer on which infected deer ticks have fed.

This introduces a completely different infection pathway that isn't part of the argument's reasoning. The argument focuses specifically on larval-stage infection from small mammals, so information about adult-stage infection from deer doesn't help evaluate the proposed intervention targeting larval feeding patterns.

Whether the other species on which deer tick larvae feed harbor any other bacteria that ticks transmits to humans.

This concerns different diseases entirely, not Lyme disease. Even if these animals carry other harmful bacteria, that wouldn't change whether increasing their populations would reduce Lyme disease specifically. The argument is focused solely on the Lyme bacterium, making information about other pathogens irrelevant to evaluating this particular intervention.

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