A 35-year-old woman presents with recurrent nephrolithiasis (calcium oxalate stones), osteomalacia and proximal renal tubular acidosis. Serum bicarbonate is 14 mEq/L, potassium 2.8 mEq/L, urine pH 5.0 after acid load test. She has nephrocalcinosis. The diagnosis is:
- A Type 1 (distal) RTA
- B Type 2 (proximal) RTA ✓
- C Type 4 RTA (hyperkalemic)
- D Lactic acidosis
Explanation
Type 2 (proximal) RTA results from defective bicarbonate reabsorption in the proximal tubule, causing bicarbonate wasting, hypokalaemia and systemic acidosis. Urine can be acidified below pH 5.5 (because distal acidification is intact) when serum bicarbonate falls below the reabsorptive threshold — this distinguishes it from type 1 (distal) RTA, where urine pH remains >5.5 despite severe acidosis. Nephrocalcinosis and stones are more characteristic of type 1 RTA; however, Fanconi syndrome (proximal RTA with phosphaturia, glycosuria, aminoaciduria) causes osteomalacia and calcium oxalate stones via hypophosphataemia. The ability to acidify urine to pH 5.0 on acid load confirms intact distal acidification and points to proximal RTA.
Reference: Harrison's Principles of Internal Medicine, 21st ed.
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