A 28-year-old woman with recurrent nephrolithiasis has persistent hypokalemic hyperchloremic metabolic acidosis: pH 7.31, bicarbonate 15 mEq/L. Urine pH measured during the acidosis is 6.8. She is not on any diuretics or alkali therapy. What is the most likely mechanism of her disorder?

• A Impaired proximal tubular reabsorption of bicarbonate
• B Failure of alpha intercalated cells to secrete hydrogen ions ✓
• C Reduced ammoniagenesis due to hyporeninemic hypoaldosteronism
• D Excessive urinary sodium loss with secondary hyperaldosteronism

Explanation

Inability to lower urine pH below 5.5 during systemic acidosis defines distal (type 1) renal tubular acidosis, caused by defective hydrogen ion secretion by the alpha intercalated cells of the collecting duct. This leads to hypokalemia, hyperchloremia, and calcium phosphate stones because alkaline urine favors phosphate precipitation. In proximal (type 2) RTA the urine can still acidify below 5.5 once plasma bicarbonate falls, which kills option A.

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

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Written and medically reviewed by the StethoPrep medical team.