A 58-year-old man with a 40 pack-year smoking history presents with progressive dyspnea on exertion. Chest X-ray shows emphysematous changes predominantly in the upper lobes. Arterial blood gas on room air reveals: pH 7.38, PaCO₂ 48 mmHg, PaO₂ 62 mmHg, HCO₃⁻ 28 mEq/L. Pulmonary function tests show FEV₁/FVC ratio of 0.52 and markedly reduced DLCO. Which ventilation-perfusion (V/Q) abnormality best explains his hypoxemia despite a relatively preserved PaCO₂?