A 35-year-old woman with a history of recurrent episodes of severe lactic acidosis (pH 7.1, lactate 8.5 mmol/L) triggered by fasting or minor infections is referred for investigation. Baseline serum lactate is 2.8 mmol/L. To confirm a defect in the Cori cycle and identify whether the problem lies in lactate production or hepatic clearance, which test is most appropriate as the initial investigative step?