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?

Explanation

## Why "Consumption of HCO3- by excess organic acids (ketoacids) in diabetic ketoacidosis" is right The clinical presentation—poorly controlled diabetes, fruity breath, altered mental status, and severe dyspnea—is classic for diabetic ketoacidosis (DKA), which is a high anion gap metabolic acidosis (HAGMA). The disorder marked **A** in the Davenport diagram represents metabolic acidosis with low HCO3- and low pH. In DKA, the primary mechanism is accumulation of unmeasured organic acids (β-hydroxybutyrate and acetoacetate) that consume bicarbonate buffer, directly lowering HCO3- concentration. The anion gap here is elevated: AG = 138 − (102 + 10) = 26 mEq/L (normal 8–12), confirming HAGMA. DKA is part of the MUDPILES mnemonic for HAGMA (M = Methanol, U = Uremia, D = DKA). The respiratory compensation (pCO2 28) approximates Winter's formula: expected pCO2 = 1.5 × 10 + 8 ± 2 = 23 ± 2 (range 21–25), indicating appropriate respiratory hyperventilation. [Harrison 21e Ch 51; Guyton & Hall 14e Ch 31] ## Why each distractor is wrong - **Renal tubular dysfunction preventing HCO3- reabsorption in the proximal tubule**: This describes renal tubular acidosis (RTA), which causes normal anion gap (hyperchloremic) metabolic acidosis. The elevated anion gap (26) rules out RTA and points to HAGMA from organic acid accumulation, not renal HCO3- wasting. - **Loss of HCO3- through gastrointestinal tract due to severe diarrhea**: Diarrhea causes normal anion gap metabolic acidosis (hyperchloremic acidosis) because bicarbonate is lost directly. The elevated anion gap in this patient excludes GI loss as the primary mechanism. - **Inhibition of carbonic anhydrase by acetazolamide leading to urinary HCO3- wasting**: Acetazolamide causes normal anion gap metabolic acidosis by blocking proximal tubule HCO3- reabsorption. Again, the elevated anion gap (26) excludes this mechanism. **High-Yield:** In metabolic acidosis marked **A**, always calculate anion gap first: HAGMA (AG > 12) suggests organic acid accumulation (MUDPILES); normal AG suggests GI loss or renal tubular dysfunction. Winter's formula validates appropriate respiratory compensation. [cite: Harrison 21e Ch 51; Guyton & Hall 14e Ch 31]