A 42-year-old woman with acute pneumonia (right lower lobe consolidation on imaging) presents with hypoxemia (PaO₂ 65 mmHg on room air, FiO₂ 0.21). ABG shows pH 7.42, PaCO₂ 38 mmHg. A-a gradient is calculated at 35 mmHg. The clinician suspects a significant right-to-left shunt component. Which investigation would best distinguish true shunt from V/Q mismatch in this patient?