A 35-year-old woman with severe diarrhea for 5 days presents with weakness and muscle cramps. Arterial blood gas shows pH 7.32, PaCO₂ 32 mmHg, HCO₃⁻ 16 mEq/L, and anion gap 12 mEq/L. Which investigation is most appropriate to differentiate the cause of the metabolic acidosis and guide fluid resuscitation?