Two patients undergo pulmonary function testing and blood gas analysis. Patient A has a V/Q ratio of 0 in 30% of lung units (true shunt), with PaO₂ 65 mmHg on room air and 68 mmHg on 100% O₂. Patient B has a V/Q ratio of 3.0 in 30% of lung units (dead space), with PaO₂ 62 mmHg on room air and 95 mmHg on 100% O₂. Which finding best distinguishes a true shunt from dead space ventilation?