A patient's arterial blood gas shows pH 7.28, PaCO2 28 mmHg, HCO3− 13 mEq/L. Which is the PRIMARY acid-base disorder and the appropriate compensatory response?

• A Respiratory acidosis with metabolic compensation
• B Mixed metabolic and respiratory acidosis
• C Respiratory alkalosis with metabolic compensation
• D Metabolic acidosis with appropriate respiratory compensation (Kussmaul breathing) ✓

Explanation

pH 7.28 (acidosis), HCO3− 13 mEq/L (low — metabolic), PaCO2 28 mmHg (low — respiratory compensation). The primary disorder is metabolic acidosis (low HCO3−). Expected PaCO2 compensation: using Winter's formula, PaCO2 = 1.5 × HCO3− + 8 ± 2 = 1.5(13) + 8 = 27.5 ± 2 = 25.5–29.5 mmHg. Measured PaCO2 28 is within the expected range, confirming appropriate respiratory compensation via Kussmaul breathing (hyperventilation driven by medullary chemoreceptors sensing low pH).

Reference: Harper's Illustrated Biochemistry, 32nd ed.

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