A patient has normal anion gap metabolic acidosis with hypokalemia. Urine pH is 6.5 (alkaline). She has a history of Sjögren's syndrome. What is the MOST likely diagnosis?
- A Type 1 (distal) renal tubular acidosis ✓
- B Type 2 (proximal) renal tubular acidosis
- C Type 4 renal tubular acidosis (hyperkalemic)
- D Diarrhea-induced metabolic acidosis
Explanation
Type 1 (distal) RTA is characterized by inability of the distal collecting duct to excrete protons (H+), resulting in alkaline urine (pH > 5.5) despite systemic acidosis—the hallmark diagnostic finding. It causes hypokalemia, hyperchloremic normal anion gap metabolic acidosis, nephrocalcinosis, and nephrolithiasis (calcium phosphate stones). Sjögren's syndrome is the most common systemic cause of distal RTA via autoimmune destruction of distal tubular intercalated cells. Type 2 RTA (proximal, as in Fanconi syndrome) also causes alkaline urine during bicarbonate wasting but urine pH becomes acidic at low serum bicarbonate levels. Type 4 RTA is hyperkalemic.
Reference: Harrison's Principles of Internal Medicine, 21st ed.
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