A 19-year-old woman with type 1 diabetes mellitus presents with Kussmaul breathing. ABG shows pH 7.18, HCO3- 10 mEq/L, PaCO2 24 mmHg. Her serum Na+ is 138 mEq/L, Cl- 102 mEq/L, and glucose 480 mg/dL. Which statement correctly describes the acid-base pathophysiology?

• A The anion gap is normal and the low PaCO2 reflects compensatory metabolic alkalosis
• B The anion gap is normal and the low PaCO2 reflects a primary respiratory alkalosis driving the acidosis
• C The anion gap is elevated and the low PaCO2 indicates a primary respiratory acidosis superimposed on metabolic acidosis
• D The anion gap is elevated and the low PaCO2 reflects appropriate respiratory compensation ✓

Explanation

In DKA, ketoacids (acetoacetate, beta-hydroxybutyrate) accumulate, producing a high anion gap metabolic acidosis. The anion gap here is 138 - (102 + 10) = 26 (elevated; normal ~12). Kussmaul breathing lowers PaCO2 as respiratory compensation (expected PaCO2 = 1.5 x HCO3- + 8 = 23 +/- 2 mmHg). Option A is wrong because the gap is elevated, not normal. Option C incorrectly labels the low PaCO2 as pathological rather than compensatory.

Reference: Harrison's Principles of Internal Medicine, 21st ed.

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