Humans get Lyme disease from infected ticks. Ticks get infected by feeding on animals with Lyme disease, but the ease...

GMAT Critical Reasoning : (CR) Questions

Source: Official Guide

Critical Reasoning

Inference

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Humans get Lyme disease from infected ticks. Ticks get infected by feeding on animals with Lyme disease, but the ease of transmission from host animal to tick varies. With most species of host animal, transmission of Lyme disease to ticks is extremely rare, but white-footed mice are an exception, readily passing Lyme disease to ticks. And white-footed mouse populations greatly expand, becoming the main food source for ticks, in areas where biodiversity is in decline.

The information in the passage most strongly supports which of the following?

In areas where many humans are infected with Lyme disease, the proportion of ticks infected with Lyme disease is especially high.

Very few animals that live in areas where there are no white-footed mice are infected with Lyme disease.

Humans are less at risk of contracting Lyme disease in areas where biodiversity is high.

Ticks feed on white-footed mice only when other host species are not available to them.

The greater the biodiversity of an area, the more likely any given host animal in that area is to pass Lyme disease to ticks.

Solution

Passage Visualization

Passage Statement	Visualization and Linkage
Humans get Lyme disease from infected ticks.	Disease Transmission Chain Established: Ticks → Humans pathway confirmed Example: If 100 ticks bite humans, infected ticks will transmit Lyme disease Key insight: Ticks are the direct vector to humans
Ticks get infected by feeding on animals with Lyme disease, but the ease of transmission from host animal to tick varies.	Variable Transmission Rates Established: Animals → Ticks pathway with variable efficiency Example: Some animals might transmit to 1% of feeding ticks, others to 90% of feeding ticks Key insight: Not all infected animals are equally effective at infecting ticks
With most species of host animal, transmission of Lyme disease to ticks is extremely rare, but white-footed mice are an exception, readily passing Lyme disease to ticks.	Transmission Efficiency Contrast: Most animals: Extremely rare transmission (maybe 1-5% success rate) White-footed mice: High transmission rate (perhaps 80-95% success rate) Example: 100 ticks feeding on deer vs. 100 ticks feeding on white-footed mice Pattern: White-footed mice are super-spreaders in the tick population
And white-footed mouse populations greatly expand, becoming the main food source for ticks, in areas where biodiversity is in decline.	Ecological Shift Pattern: Biodiversity decline → White-footed mouse population explosion White-footed mice become dominant tick food source Example: Area with 20 animal species → declines to 5 species → white-footed mice go from 10% to 70% of tick meals Critical insight: Biodiversity loss concentrates tick feeding on the most infectious host
Overall Implication	BIODIVERSITY PARADOX REVEALED: As biodiversity decreases → More white-footed mice (super-spreaders) → More infected ticks → Higher Lyme disease risk to humans Counter-intuitive result: Less biological diversity = More disease risk

Valid Inferences

Inference: Areas with declining biodiversity will likely have higher rates of Lyme disease transmission to humans.

Supporting Logic: Since white-footed mice are exceptionally effective at transmitting Lyme disease to ticks, and since these mice become the dominant food source for ticks in areas where biodiversity is declining, ticks in such areas will have much higher infection rates. Since humans get Lyme disease from infected ticks, areas with declining biodiversity will pose greater Lyme disease risks.

Clarification Note: The passage supports this inference about the relationship between biodiversity and disease risk, but does not explain why white-footed mice populations expand when biodiversity declines or why they are such effective disease transmitters.

Answer Choices Explained

In areas where many humans are infected with Lyme disease, the proportion of ticks infected with Lyme disease is especially high.

While this might seem logical, the passage doesn't provide information about areas where many humans are already infected. We can't work backwards from human infection rates to determine tick infection rates based on the given information. The passage tells us how the disease flows from animals to ticks to humans, but doesn't support inferences about areas with high human infection rates.

Very few animals that live in areas where there are no white-footed mice are infected with Lyme disease.

This goes too far. The passage tells us that transmission from most animals to ticks is extremely rare, but it doesn't tell us anything about the infection rates in the animals themselves. We know white-footed mice are good at transmitting the disease to ticks, but this doesn't mean other animals can't be infected with Lyme disease - they might just be poor at passing it to ticks.

Humans are less at risk of contracting Lyme disease in areas where biodiversity is high.

This is strongly supported by the logical chain the passage provides. High biodiversity means white-footed mice don't dominate as the tick food source. Since white-footed mice are the primary effective transmitters of Lyme disease to ticks, areas with high biodiversity will have fewer infected ticks. With fewer infected ticks, humans face lower risk of contracting Lyme disease. This creates a clear inverse relationship between biodiversity and human risk.

Ticks feed on white-footed mice only when other host species are not available to them.

The passage doesn't suggest that ticks prefer other species over white-footed mice. It simply states that when biodiversity declines, white-footed mouse populations expand and become the main food source. This could be due to mouse population growth rather than tick feeding preferences.

The greater the biodiversity of an area, the more likely any given host animal in that area is to pass Lyme disease to ticks.

This directly contradicts the passage. The passage tells us that white-footed mice are exceptional transmitters while most other species transmit extremely rarely. If biodiversity is high, we'd have more of these 'most other species' that are poor transmitters, making the average transmission rate lower, not higher.

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