A 25-year-old woman presents with recurrent nephrolithiasis (calcium oxalate stones) and is found to have serum potassium 3.1 mEq/L, urine pH 6.5 (inappropriately alkaline), hyperchloraemic metabolic acidosis, and urine anion gap of +10. Serum bicarbonate is 15 mEq/L. What is the diagnosis and underlying defect?
- A Proximal RTA (Type 2): bicarbonate wasting by defective proximal tubule reabsorption
- B Distal RTA (Type 1): inability of alpha-intercalated cells to secrete H+ ion despite acidaemia ✓
- C Type 4 RTA: hyporeninism-hypoaldosteronism causing hyperkalemia and impaired ammoniagenesis
- D Diarrhoea-induced metabolic acidosis with urinary potassium wasting
Explanation
Distal (Type 1) RTA is characterised by: (1) inability to acidify urine below pH 5.5 despite systemic acidosis; (2) hypokalaemia (H+ secretion failure drives K+ wasting); (3) positive urine anion gap (UAG = urine [Na+K] – [Cl] > 0, reflecting impaired NH4+ excretion); and (4) calcium phosphate and oxalate nephrolithiasis from alkaline urine and hypercalciuria. Proximal RTA (Type 2) would show urine pH <5.5 once threshold is exceeded and would present with bicarbonaturia, glycosuria, phosphaturia (Fanconi syndrome). Type 4 RTA shows hyperkalemia (not hypokalemia). Diarrhoea would have negative UAG (reflecting appropriate ammonium excretion).
Reference: Harrison's Principles of Internal Medicine, 21st ed.
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Written and medically reviewed by the StethoPrep medical team.