Neuroscience MCAT Practice Question
A 68-year-old woman presents with progressive memory loss and cognitive decline over the past 2 years. Brain MRI shows cerebral atrophy and her cerebrospinal fluid analysis reveals elevated phosphorylated tau and decreased amyloid-beta-42 levels. A positron emission tomography scan confirms amyloid and tau pathology consistent with Alzheimer's disease. Neuropathological examination would likely reveal extracellular amyloid-beta plaques composed of aggregated peptides derived from sequential proteolytic cleavage of amyloid precursor protein. Which of the following best explains the preferential accumulation and aggregation of amyloid-beta in the extracellular space rather than remaining as soluble monomers?
Answer choices
- AAmyloid-beta is predominantly hydrophilic and forms hydrogen bonds with extracellular water molecules, which paradoxically promotes beta-sheet formation and aggregation
- BThe extracellular environment has a pH approximately 0.5 units lower than intracellular compartments, which protonates amyloid-beta and neutralizes its charge
- CAmyloid-beta contains clustered hydrophobic amino acid residues that self-associate to minimize thermodynamically unfavorable contact with the aqueous extracellular milieuCorrect answer
- DExtracellular proteases such as neprilysin and insulin-degrading enzyme actively cross-link amyloid-beta monomers into ordered polymeric structures
- EApolipoprotein E and other extracellular chaperone proteins sequester amyloid-beta through specific binding interactions that facilitate ordered crystalline aggregate formation
- FReactive oxygen species generated in the extracellular space form disulfide bonds between amyloid-beta molecules, covalently linking them into stable higher-order complexes
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