A 68-year-old male with COPD (FEV₁ 45% predicted) and coronary artery disease undergoes emergency repair of perforated peptic ulcer. Induction is uneventful, but 15 minutes after intubation, SpO₂ drops to 85% despite FiO₂ 1.0 and manual ventilation at 15 L/min. Auscultation reveals reduced breath sounds bilaterally with scattered crackles. Peak airway pressure has risen from 18 to 32 cm H₂O. The anesthesiologist suspects acute lung injury secondary to aspiration or sepsis. Which single investigation is most specific for confirming aspiration as the cause?