The milk of many mammals contains cannabinoids, substances that are known to stimulate certain receptors in the brain. To investigate...

GMAT Critical Reasoning : (CR) Questions

Source: Official Guide

Critical Reasoning

Assumption

MEDIUM

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The milk of many mammals contains cannabinoids, substances that are known to stimulate certain receptors in the brain. To investigate the function of cannabinoids, researchers injected newborn mice with a chemical that is known to block cannabinoids from reaching their receptors in the brain. The injected mice showed far less interest in feeding than normal newborn mice do. Therefore, cannabinoids probably function to stimulate the appetite.

Which of the following is an assumption on which the argument depends?

Newborn mice do not normally ingest any substance other than their mothers' milk.

Cannabinoids are the only substances in mammals' milk that stimulate the appetite.

The mothers of newborn mice do not normally make any effort to encourage their babies to feed.

The milk of mammals would be less nutritious if it did not contain cannabinoids.

The chemical that blocks cannabinoids from stimulating their brain receptors does not independently inhibit the appetite.

Solution

Passage Analysis:

Text from Passage	Analysis
The milk of many mammals contains cannabinoids, substances that are known to stimulate certain receptors in the brain.	What it says: Mammal milk has cannabinoids that affect brain receptors What it does: Sets up basic scientific background about cannabinoids What it is: Scientific fact/background information Visualization: Many mammals (let's say 25 out of 30) → milk contains cannabinoids → brain receptors
To investigate the function of cannabinoids, researchers injected newborn mice with a chemical that is known to block cannabinoids from reaching their receptors in the brain.	What it says: Scientists blocked cannabinoids in baby mice to study what they do What it does: Introduces the experiment design, building on the cannabinoid background What it is: Study methodology Visualization: Normal mice: cannabinoids → brain receptors ✓ Test mice: cannabinoids ✗ (blocked) brain receptors
The injected mice showed far less interest in feeding than normal newborn mice do.	What it says: Mice without cannabinoids ate much less than normal mice What it does: Reveals the key experimental result that connects to the previous setup What it is: Study findings Visualization: Normal mice feeding interest: 80% Blocked cannabinoid mice feeding interest: 20%
Therefore, cannabinoids probably function to stimulate the appetite.	What it says: The author concludes cannabinoids make mammals want to eat What it does: Draws a conclusion from the experimental evidence presented What it is: Author's conclusion

Argument Flow:

The argument starts with background info about cannabinoids, then describes an experiment where researchers blocked these substances in mice. When the mice couldn't get cannabinoids, they ate less. From this result, the author concludes that cannabinoids must stimulate appetite.

Main Conclusion:

Cannabinoids probably function to stimulate the appetite.

Logical Structure:

The logic connects the experimental evidence (blocked cannabinoids = less eating) to the conclusion (cannabinoids stimulate appetite). The author assumes that if removing something causes less eating, then that something must promote eating.

Prethinking:

Question type:

Assumption - We need to find what the argument must assume to be true for the conclusion to work. These are hidden beliefs that make the logic hold together.

Precision of Claims

The argument makes specific claims about cannabinoids' function based on experimental evidence - mice without cannabinoids showed 'far less interest in feeding' leading to the conclusion that cannabinoids 'stimulate appetite'.

Strategy

For assumption questions, we identify ways the conclusion could fall apart while keeping all the stated facts true. The argument concludes cannabinoids stimulate appetite because blocking them reduced feeding interest. We need to find what gaps exist between the evidence (reduced feeding interest) and conclusion (appetite stimulation).

Answer Choices Explained

Newborn mice do not normally ingest any substance other than their mothers' milk.

This states that newborn mice only consume their mothers' milk and nothing else. While this might strengthen the argument slightly, it's not something the argument must assume. The argument is about the effect of blocking cannabinoids, not about what else mice might eat. Even if mice ate other things, the experimental design (blocking cannabinoids vs. not blocking them) would still be valid for comparing feeding interest.

Cannabinoids are the only substances in mammals' milk that stimulate the appetite.

This claims cannabinoids are the only appetite-stimulating substances in mammal milk. This is way too strong and not required. The argument only concludes that cannabinoids probably function to stimulate appetite - it doesn't need to assume they're the only substance that does this. Other substances in milk could also stimulate appetite without undermining the conclusion about cannabinoids.

The mothers of newborn mice do not normally make any effort to encourage their babies to feed.

This suggests mother mice don't encourage their babies to feed. This isn't necessary for the argument because the experiment compares two groups of mice (blocked vs. normal cannabinoids) under presumably similar conditions. Whether mothers encourage feeding or not would affect both groups equally, so it doesn't impact the comparison that leads to the conclusion.

The milk of mammals would be less nutritious if it did not contain cannabinoids.

This states milk would be less nutritious without cannabinoids. The argument is about appetite stimulation, not nutrition. Cannabinoids could stimulate appetite without affecting nutritional value, and the experimental evidence (reduced feeding interest when cannabinoids are blocked) supports the appetite conclusion regardless of nutritional considerations.

The chemical that blocks cannabinoids from stimulating their brain receptors does not independently inhibit the appetite.

This is the correct answer. The argument concludes that cannabinoids stimulate appetite because blocking them reduced feeding interest. But we must assume the blocking chemical itself doesn't suppress appetite. If the chemical independently inhibited appetite, then the reduced feeding could be caused by the chemical directly rather than by the absence of cannabinoids. This assumption is essential - without it, the experimental results don't support the conclusion about cannabinoids' function.

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