A 35-year-old woman with recurrent kidney stones is found to have normal anion gap metabolic acidosis, hypokalemia, urine pH 6.2 despite systemic acidosis, and urinary Ca²⁺ >250 mg/day. Which type of renal tubular acidosis (RTA) is most consistent?
- A Type 2 (proximal) RTA — defective HCO3⁻ reclamation in PCT causing HCO3⁻ wasting
- B Type 4 RTA — hypoaldosteronism causing hyperkalemia and impaired NH4⁺ excretion
- C Type 3 RTA — combined proximal and distal H⁺ secretion defect
- D Type 1 (distal) RTA — inability to lower urine pH below 5.5 due to defective H⁺ secretion in collecting duct alpha-intercalated cells ✓
Explanation
Type 1 (distal) RTA results from failure of alpha-intercalated cells to secrete H⁺ in the collecting duct. Key features: (1) inability to acidify urine below pH 5.5 (here 6.2) despite systemic acidosis; (2) normal anion gap hyperchloremic metabolic acidosis; (3) hypokalemia (due to K⁺/H⁺ exchange — as H⁺ cannot be excreted, K⁺ is secreted instead in the collecting duct, plus aldosterone activation); (4) nephrocalcinosis and nephrolithiasis due to hypercalciuria (acidosis mobilizes bone calcium) and hypocitraturia (citrate excretion falls in acidic urine, reducing chelation of Ca²⁺). Type 2 RTA has urine pH <5.5 when plasma HCO3⁻ falls below threshold (correct acidification distal mechanism). Type 4 causes hyperkalemia, not hypokalemia.
Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.
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