A 40-year-old woman with a history of recurrent renal calculi has serum: Na 139, K 2.9, Cl 114, HCO3 14 mmol/L, albumin normal. Urine pH is 6.5. Urine anion gap is +12 mEq/L (positive). Which type of renal tubular acidosis (RTA) does this represent?
- A Type 1 RTA (distal, hypokalemic) ✓
- B Type 2 RTA (proximal)
- C Type 4 RTA (hyperkalemic, hyporeninemic hypoaldosteronism)
- D Type 3 RTA (combined proximal and distal)
Explanation
Type 1 (distal) RTA is characterised by failure to acidify urine below pH 5.5, leading to a normal anion gap hyperchloraemic metabolic acidosis, hypokalaemia (due to urinary potassium wasting), and nephrolithiasis/nephrocalcinosis (calcium phosphate stones). Urine pH persistently >5.5 even with systemic acidosis and a positive urine anion gap (reflecting low urinary ammonium, as UAG = Na+K-Cl; positive means low NH4+) confirm failure of H+ secretion in the collecting duct. Type 2 RTA has urine pH that can drop below 5.5 once HCO3 threshold is surpassed. Type 4 RTA has hyperkalemia, not hypokalemia.
Reference: Harrison's Principles of Internal Medicine, 21st ed.
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