A 35-year-old woman presents with recurrent renal stones and serum chemistry showing hyperchloremic metabolic acidosis with normal anion gap, serum K+ 2.9 mEq/L, and urine pH 6.8 despite serum bicarbonate of 16 mEq/L. The diagnosis is:
- A Type 1 (distal) RTA — inability to acidify urine distally, urine pH persistently >5.5 ✓
- B Type 2 (proximal) RTA — proximal bicarbonate wasting with urine pH <5.5 when acidotic
- C Type 4 RTA — hyperkalemic hyperchloremic acidosis from aldosterone resistance
- D Type 3 (combined) RTA — carbonic anhydrase inhibitor effect
Explanation
Type 1 (distal) RTA is characterized by the inability of the collecting tubule to maintain a proton gradient, resulting in persistently alkaline urine (pH >5.5) even during systemic acidosis. Key features include hyperchloremic normal anion gap metabolic acidosis, hypokalemia (aldosterone-driven K+ loss to compensate for H+ retention), nephrocalcinosis and nephrolithiasis (calcium phosphate stones, due to hypercalciuria, hypocitraturia, and alkaline urine). Type 2 RTA causes HCO3 wasting proximally but can still acidify urine (pH <5.5) when serum HCO3 falls below threshold — urine pH is acid in established acidosis. Type 4 causes hyperkalemia not hypokalemia.
Reference: Harrison's Principles of Internal Medicine, 21st ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.