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PaO2 100% Oxygen Expected — FRCA Primary MCQ

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HardPhysiology - RespiratoryPaO2 100% Oxygen ExpectedFRCA Primary

At sea level, a patient breathing room air has PAO2 calculated as 13.3 kPa and measured PaO2 of 8.0 kPa. Which conclusion is most appropriate before accounting for age?

Educational content. Not a substitute for clinical judgement or local policy.

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Correct answer: E — The alveolar–arterial oxygen gradient is widened by approximately 5.3 kPa

The best answer is “The alveolar–arterial oxygen gradient is widened by approximately 5.3 kPa”. The A–a gradient is PAO2 minus PaO2, here approximately 5.3 kPa, which is wider than expected in a young healthy adult and suggests V/Q mismatch, diffusion impairment or shunt. A positive gradient is normal in principle but magnitude matters. It does not uniquely determine respiratory quotient, PaCO2 or shunt fraction.

Reference: RCoA, Primary FRCA syllabus: https://www.rcoa.ac.uk/sites/default/files/documents/2026-01/Primary-FRCA-Syllabus.pdf; The respiratory system and acid–base disorders: https://pmc.ncbi.nlm.nih.gov/articles/PMC10201398/