A 58-year-old man with chronic obstructive pulmonary disease (COPD) presents with progressive dyspnea. Arterial blood gas analysis shows PaO₂ 65 mmHg, PaCO₂ 50 mmHg, and pH 7.32. Chest X-ray is unremarkable. Which investigation is most appropriate to quantify the degree of ventilation-perfusion (V/Q) mismatch and guide therapeutic decisions?

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