A 35-year-old man with poorly controlled diabetes presents to the emergency department with severe dyspnea, fruity-smelling breath, and altered mental status. Arterial blood gas shows pH 7.15, pCO2 28 mmHg, HCO3- 10 mEq/L, and serum electrolytes reveal Na+ 138, Cl- 102, HCO3- 10. The acid-base disorder marked **A** in the Davenport diagram is confirmed. Which of the following is the PRIMARY pathophysiologic mechanism responsible for the low bicarbonate concentration in this patient's presentation?