A 28-year-old man with a 3-day history of severe vomiting and abdominal pain is admitted with suspected starvation ketoacidosis. His serum glucose is 85 mg/dL, pH is 7.28, and HCO₃⁻ is 14 mEq/L. Which investigation would be most appropriate to differentiate starvation ketoacidosis from diabetic ketoacidosis and assess the predominant ketone body present?