5

Set 2: Central Ideas (Intermediate)

Explanation

Answer: B

PASSAGE

The following text describes the function of the blood-brain barrier. The blood-brain barrier (BBB) is a highly selective semipermeable border of endothelial cells that prevents solutes in the circulating blood from non-selectively crossing into the extracellular fluid of the central nervous system where neurons reside. The BBB allows the passage of water, some gases, and lipid-soluble molecules by passive diffusion, as well as the selective transport of molecules such as glucose and amino acids that are crucial to neural function.

What is the primary function of the blood-brain barrier?

A. To prevent any substances from leaving the brain
B. To selectively control what enters the central nervous system✓ Correct
C. To produce glucose and amino acids for neurons
D. To pump blood directly into the neurons

Detailed Explanation

Choice B is correct. It is described as a 'highly selective... border' that 'prevents... non-selective crossing' while allowing 'selective transport' of crucial molecules.

Key Evidence:

• "highly selective semipermeable border"

• "prevents solutes... from non-selectively crossing"

• "selective transport of molecules"

Why others are wrong: A (Inaccurate (it regulates entry, doesn't just block exit)), C (Inaccurate (allows passage, does not produce)), D (Inaccurate).

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