A 35-year-old woman presents with fatigue, bone pain, and recurrent nephrolithiasis. Labs: serum Na 138, K 2.8 mEq/L, Cl 112 mEq/L, HCO₃ 14 mEq/L, anion gap = 12. Urine pH is 6.2. Serum potassium is low. Urine anion gap is positive (+8). Which type of renal tubular acidosis (RTA) is most consistent with this picture?

• A Type 2 RTA (Proximal RTA)
• B Type 4 RTA (Hyporeninemic hypoaldosteronism)
• C Normal anion gap acidosis due to diarrhea
• D Type 1 RTA (Distal RTA) ✓

Explanation

Type 1 (distal) RTA results from inability of collecting duct intercalated cells to secrete H⁺, causing failure to acidify urine below pH 6.0 despite systemic acidosis. Key features: hyperchloremic, normal anion gap metabolic acidosis; urine pH persistently > 5.5; hypokalemia (from urinary K⁺ wasting); nephrocalcinosis and nephrolithiasis due to calcium phosphate stones from hypercalciuria and hypocitraturia; positive urine anion gap. Type 4 RTA presents with hyperkalemia; Type 2 has urine pH < 5.5 during acidosis.

Reference: Harrison's Principles of Internal Medicine, 21st ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.