A 38-year-old female with no significant past medical history undergoes emergency laparotomy for acute abdomen. Intraoperatively, 45 minutes after induction with thiopental and vecuronium, she develops acute hypotension (BP 70/40 mmHg), tachycardia (HR 130/min), flushed skin, and rigid abdominal muscles. Capnography shows a rising end-tidal CO₂ (6.5 kPa). Which investigation is most specific for confirming the suspected diagnosis?