A 30-year-old woman presents with recurrent renal stones (calcium phosphate) and osteoporosis. ABG: pH 7.30, PCO2 32 mmHg, HCO3 16 mEq/L. Urine pH is 6.5 despite systemic acidosis. Serum potassium is 2.9 mEq/L. Serum calcium is 10.6 mg/dL. What is the most likely diagnosis?
- A Type 2 (proximal) renal tubular acidosis
- B Type 1 (distal) renal tubular acidosis ✓
- C Type 4 (hyperkalemic) renal tubular acidosis
- D Hyperparathyroidism-induced acidosis
Explanation
Type 1 (distal) RTA is characterized by inability to acidify urine below pH 5.5 even during systemic acidosis (urine pH >5.5 with blood pH <7.35), hypokalemia (due to urinary potassium wasting from impaired H+ secretion), hyperchloremic normal-anion-gap metabolic acidosis, hypercalciuria, nephrocalcinosis, nephrolithiasis (calcium phosphate stones — alkaline urine precipitates CaPO4), and osteoporosis/osteomalacia. Type 2 RTA shows urine pH that can reach <5.5 during severe acidosis, with bicarbonaturia and hypokalemia. Type 4 is hyperkalemic and associated with aldosterone deficiency.
Reference: Harrison's Principles of Internal Medicine, 21st ed.
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