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Blood-Brain Barrier — FRCA Primary MCQ

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HardPhysiology - NeurophysiologyBlood-Brain BarrierFRCA Primary

A hydrophilic quaternary ammonium drug has negligible central nervous-system effect despite a high plasma concentration. Which blood–brain-barrier feature most directly limits its passive entry?

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Correct answer: A — Tight cerebral endothelial junctions with poor ion diffusion

The best answer is “Tight cerebral endothelial junctions with poor ion diffusion”. Brain capillary endothelial tight junctions restrict paracellular movement, while a permanently charged quaternary ammonium compound crosses cell membranes poorly by passive diffusion. Cerebral capillaries are not generally fenestrated. Circumventricular organs such as the area postrema are exceptions, not a description of the barrier throughout the brain.

Reference: RCoA, Primary FRCA syllabus: https://www.rcoa.ac.uk/sites/default/files/documents/2026-01/Primary-FRCA-Syllabus.pdf; Drug transport across the blood–brain barrier: https://pmc.ncbi.nlm.nih.gov/articles/PMC3494002/