A patient with chronic diarrhea develops hyperchloremic metabolic acidosis with normal anion gap. What is the BIOCHEMICAL explanation for the normal anion gap?

• A Diarrhea causes loss of Cl− ions, reducing the anion gap
• B HCO3− lost in diarrheal fluid is replaced by Cl− retention in kidneys, maintaining electroneutrality without accumulating unmeasured anions ✓
• C Lactate accumulates to fill the anion gap
• D Potassium depletion from diarrhea directly affects anion gap calculation

Explanation

Diarrheal fluid is rich in HCO3− (up to 40–60 mEq/L in secretory diarrhea). As HCO3− is lost, serum HCO3− falls, causing metabolic acidosis. To maintain electrochemical neutrality with reduced HCO3− levels, kidneys increase Cl− reabsorption (compensating for lost bicarbonate equivalents), resulting in hyperchloremia. The anion gap = Na+ − (Cl− + HCO3−) remains normal (8-12 mEq/L) because both Cl− rises proportionally to HCO3− falls — no unmeasured anions accumulate.

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

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