A 52-year-old woman with cirrhosis (Child-Pugh B) presents with persistent fatigue and muscle weakness. Blood lactate is 4.2 mmol/L (normal <2 mmol/L), pyruvate is 0.3 mmol/L (normal 0.03–0.1 mmol/L), and lactate:pyruvate ratio is 14:1 (normal <10:1). Arterial pH is 7.32, HCO₃⁻ is 18 mEq/L. Liver synthetic function is mildly impaired. What is the most appropriate next step in management?