A patient with pyloric stenosis secondary to peptic ulcer disease has protracted vomiting. ABG shows pH 7.52, PaCO2 48 mmHg, HCO3− 38 mEq/L. Urine pH is 5.0 (paradoxical aciduria). What explains the paradoxical aciduria?
- A Renal tubular acidosis superimposed on metabolic alkalosis
- B Excess HCO3− overwhelms renal reabsorptive capacity causing alkaline urine
- C Volume depletion and hypokalemia cause the kidney to preferentially excrete H+ to preserve Na+ and K+ ✓
- D Respiratory alkalosis drives urinary acidification via carbonic anhydrase
Correct answer: C. Volume depletion and hypokalemia cause the kidney to preferentially excrete H+ to preserve Na+ and K+
Explanation
In protracted vomiting, volume depletion activates the renin-angiotensin-aldosterone system, driving Na+ reabsorption in exchange for H+ and K+ secretion. Concomitant hypokalemia causes intracellular K+ to shift out of cells while H+ moves in, further acidifying tubular cells and promoting H+ secretion. The net result is the kidney excretes acid urine (paradoxical aciduria) even though systemic alkalosis persists — called 'contraction alkalosis maintaining paradoxical aciduria.' This resolves only with volume and KCl replacement.
Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.
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