A 68-year-old man develops sudden pleuritic chest pain and tachypnoea after prolonged immobility. Arterial blood gas analysis on room air shows pH 7.48, PaCO2 3.7 kPa (reference 4.7–6.0 kPa) and PaO2 8.0 kPa (reference 10.7–13.3 kPa). His chest radiograph is clear, and ventilation is preserved in several regions with markedly reduced perfusion. Which physiological change is most responsible for the gas-exchange abnormality?
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