Lyme disease is caused by a bacterium transmitted to people by deer ticks. Deer ticks are almost never born infected...

GMAT Critical Reasoning : (CR) Questions

Source: Official Guide

Critical Reasoning

Logically Completes

HARD

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Notes

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Lyme disease is caused by a bacterium transmitted to people by deer ticks. Deer ticks are almost never born infected with the bacterium, but they can pick it up from feeding on an infected host in any of the tick's three successive stage of development: larva, nymph, adult. Once infected, a tick remains infected through any subsequent developmental stages, but since a tick feeds on only one host in each stage, it follows that

Which of the following most logically completes the argument ?

People can be infected only by being bitten by a tick that is either in larval stage or in the nymph stage

People cannot be infected by a tick that picked up the Lyme disease bacterium in the adult stage

It is not possible for a person to be infected by a tick that is in the nymph stage or by a tick that was infected with the bacterium while in the nymph stage

People infected with the Lyme disease bacterium cannot transmit bacterium to adult deer ticks

Only deer ticks that are adults can transmit the Lyme disease bacterium to people

Solution

Passage Analysis:

Text from Passage	Analysis
Lyme disease is caused by a bacterium transmitted to people by deer ticks.	What it says: Lyme disease comes from bacteria that deer ticks pass to humans What it does: Sets up the basic cause-and-effect relationship we need to understand What it is: Author's factual premise
Deer ticks are almost never born infected with the bacterium, but they can pick it up from feeding on an infected host in any of the tick's three successive stage of development: larva, nymph, adult.	What it says: Ticks aren't born with the bacteria, but can get infected while feeding at any of their 3 life stages What it does: Explains how ticks become infected carriers, building on the transmission concept What it is: Author's factual premise Visualization: Tick Life Cycle: Larva → Nymph → Adult (can get infected at any stage)
Once infected, a tick remains infected through any subsequent developmental stages, but since a tick feeds on only one host in each stage, it follows that	What it says: Infected ticks stay infected as they grow, and each tick only feeds once per life stage What it does: Introduces a key constraint (one feeding per stage) that will lead to a logical conclusion What it is: Author's premise leading to conclusion Visualization: Each tick: 3 stages = 3 different hosts maximum (one host per stage)

Argument Flow:

The argument starts with basic facts about Lyme disease transmission, then explains how ticks get infected, and finally sets up key constraints about tick feeding patterns to reach a logical conclusion about infection transmission.

Main Conclusion:

The argument is incomplete - we need to determine what logically follows from the given premises about tick feeding and infection patterns.

Logical Structure:

The premises establish that ticks can only get infected by feeding on infected hosts, they stay infected once infected, and they feed only once per life stage. These constraints should lead to a logical conclusion about how infection spreads or when ticks can transmit the disease.

Prethinking:

Question type:

Logically Completes - We need to find what conclusion logically follows from the given premises about how ticks get infected and their feeding patterns.

Precision of Claims

The claims are very specific about tick behavior: they feed only once per developmental stage, can get infected at any stage, and stay infected once they pick up the bacterium. We need a conclusion that directly follows from these precise constraints.

Strategy

Since this is a 'logically completes' question, we need to trace through the logical chain step by step. We know: (1) ticks feed once per stage, (2) they have 3 stages, (3) they can get infected at any stage, (4) once infected they stay infected. The conclusion should follow inevitably from these facts about the maximum number of hosts a tick can potentially infect or the transmission patterns that result from these feeding limitations.

Answer Choices Explained

People can be infected only by being bitten by a tick that is either in larval stage or in the nymph stage

This suggests people can only be infected by larval or nymph stage ticks, but this misses a crucial point. Adult ticks that became infected in earlier stages (larva or nymph) can still transmit the disease to people when they feed as adults. The passage tells us infected ticks remain infected through subsequent stages, so an adult tick that got infected as a larva or nymph is still dangerous. This choice incorrectly eliminates adult ticks entirely.

People cannot be infected by a tick that picked up the Lyme disease bacterium in the adult stage

This perfectly captures the logical conclusion. If a tick picks up the Lyme disease bacterium during its adult stage feeding, it cannot transmit the disease to people because it has already used its one feeding opportunity for that stage. Since adult is the final developmental stage, the tick has no future feeding opportunities to pass on the infection. This follows directly from the 'one feeding per stage' constraint established in the passage.

It is not possible for a person to be infected by a tick that is in the nymph stage or by a tick that was infected with the bacterium while in the nymph stage

This choice makes two claims that contradict the passage. First, it says people cannot be infected by nymph stage ticks, but the passage suggests ticks can transmit disease at any stage if they're already infected. Second, it eliminates ticks that became infected as nymphs, but these ticks would remain infected as adults and could still transmit disease during their adult feeding. This choice is too restrictive and doesn't follow from the given premises.

People infected with the Lyme disease bacterium cannot transmit bacterium to adult deer ticks

This introduces information about people transmitting bacteria back to adult ticks, but the passage doesn't provide any premises about reverse transmission from humans to ticks. The argument focuses on how ticks get infected and when they can transmit disease, not on whether infected people can infect ticks. This choice goes beyond what we can conclude from the given information.

Only deer ticks that are adults can transmit the Lyme disease bacterium to people

This incorrectly restricts transmission to adult ticks only. However, infected nymph ticks can certainly transmit the disease to people during their nymph stage feeding. The passage establishes that ticks remain infected through subsequent stages, which means they're infected and capable of transmission immediately after picking up the bacterium, not just in the adult stage. This choice contradicts the transmission possibilities described in the passage.

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