A 22-year-old woman presents with recurrent urolithiasis and low back pain. Arterial blood gas: pH 7.30, HCO3 12 mEq/L, pCO2 28 mmHg, Na+ 138, K+ 2.8 mEq/L, Cl- 112 mEq/L. Urine pH is 6.8 despite systemic acidosis. Urine anion gap is positive (+14). The most likely diagnosis is:
- A Type 2 (proximal) renal tubular acidosis
- B Type 4 RTA (hypoaldosteronism)
- C Diarrhea-related metabolic acidosis
- D Type 1 (distal) renal tubular acidosis ✓
Explanation
The hallmark of distal (Type 1) RTA is the inability to acidify urine below pH 5.5 despite systemic metabolic acidosis — urine pH >5.5 in the setting of acidosis is pathognomonic. Type 1 RTA presents with hypokalemia (urinary K+ wasting), nephrocalcinosis, and nephrolithiasis (calcium phosphate stones due to persistently alkaline urine). Urine anion gap (UAG = Na+ + K+ - Cl-) is positive in RTA (impaired NH4+ excretion). In diarrhea, UAG is negative (normal NH4+ excretion). Type 2 RTA causes acidic urine when serum HCO3 falls below tubular threshold. Type 4 causes hyperkalemia, not hypokalemia.
Reference: Harrison's Principles of Internal Medicine, 21st ed.
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Written and medically reviewed by the StethoPrep medical team.