A 35-year-old woman has recurrent nephrolithiasis (calcium oxalate stones), nephrocalcinosis, and an arterial blood gas showing: pH 7.30, pCO2 40 mmHg, HCO3 18 mEq/L, urine pH 6.8 despite systemic acidosis. Serum potassium is 2.8 mEq/L. What is the diagnosis?
- A Type 2 (proximal) RTA
- B Type 1 (distal) RTA ✓
- C Type 4 (hyperkalemic) RTA
- D Diarrhea-induced metabolic acidosis
Correct answer: B. Type 1 (distal) RTA
Explanation
Type 1 (distal) RTA is characterized by inability to acidify urine below pH 5.5, resulting in inappropriately alkaline urine (pH >6) despite systemic acidosis, hypokalemia, nephrolithiasis (calcium phosphate stones in alkaline urine), and nephrocalcinosis. The high urine pH in the presence of metabolic acidosis is the hallmark. Type 2 RTA shows urine pH <5.5 when the bicarbonate threshold is exceeded and produces proximal tubular dysfunction (Fanconi syndrome). Type 4 RTA shows hyperkalemia, not hypokalemia.
Reference: Harrison's Principles of Internal Medicine, 21st ed.
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