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Difficulty: MediumRelationships Between Sections and Paragraphs

The following passage is adapted from an article about neuroanatomy.

Paragraph 1
Historically, neuroscientists believed that the brain, unlike other organs in the human body, lacked a dedicated lymphatic system for clearing metabolic waste. Since the brain consumes a vast amount of energy and produces significant cellular debris, the absence of such a system was a long-standing anatomical puzzle. For decades, researchers assumed that waste clearance occurred slowly through the passive diffusion of cerebrospinal fluid.

Paragraph 2
This passive-diffusion model was challenged in 2012 when researchers identified a specialized waste clearance pathway they termed the "glymphatic system." Utilizing advanced two-photon imaging in living mice, scientists observed that cerebrospinal fluid is actively pumped through the brain’s tissues, flushing out toxic proteins like amyloid-beta. This process, facilitated by glial cells, operates primarily during deep sleep, suggesting a functional link between sleep quality and cognitive health.

Which of the following best describes the relationship between Paragraph 1 and Paragraph 2?

  1. A
    Paragraph 1 establishes a proven anatomical fact, while Paragraph 2 proposes a hypothetical explanation that remains unverified.
  2. Paragraph 1 outlines a long-held scientific assumption, while Paragraph 2 describes the discovery that challenged and refined that assumption.Answer
  3. C
    Paragraph 1 introduces a theoretical solution to a biological puzzle, while Paragraph 2 presents the experimental evidence that disproved this solution.
  4. D
    Paragraph 1 details the metabolic waste products of the human body, while Paragraph 2 focuses specifically on the role of sleep in brain health.

Answer

Paragraph 1 outlines a long-held scientific assumption, while Paragraph 2 describes the discovery that challenged and refined that assumption.
The correct answer accurately identifies the structural relationship: Paragraph 1 outlines the long-held assumption that the brain lacks an active waste clearance system and relies on passive diffusion, while Paragraph 2 introduces the discovery of the glymphatic system, which actively pumps cerebrospinal fluid and directly challenged the passive-diffusion assumption.

Step-by-Step Solution

1
Analyze the main idea and function of Paragraph 1.
Paragraph 1 discusses the historical belief that the brain cleared waste via passive diffusion due to the lack of a dedicated lymphatic system, which served as a long-standing assumption in neuroscience.
Identifying the baseline scientific understanding presented at the start of the passage is necessary to track how the text develops.
2
Analyze the main idea and function of Paragraph 2.
Paragraph 2 introduces the 2012 discovery of the glymphatic system, which revealed that cerebrospinal fluid is actively pumped to clear waste, showing that the passive-diffusion model was incorrect.
Understanding the new information allows for a comparison with the historical perspective established in the previous paragraph.
3
Determine the structural relationship between the two paragraphs and evaluate the options.
The transition from Paragraph 1 to Paragraph 2 shifts the focus from an old assumption to a new discovery that challenged it, making the option describing this shift the correct choice.
The relationship is defined by a transition of contrast and development where new empirical evidence refutes a historical model.

Key Concept

Relationships Between Sections and Paragraphs
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Topics

Relationships Between Sections and Paragraphs

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