A 28-year-old woman presents with recurrent nephrolithiasis (calcium phosphate stones), hypokalemia, and a non-anion gap metabolic acidosis. Urine pH is 6.5 despite systemic acidosis. Urinary anion gap is positive. What is the most likely diagnosis?

• A Type 2 (proximal) renal tubular acidosis (pRTA)
• B Type 1 (distal) renal tubular acidosis (dRTA) ✓
• C Type 4 renal tubular acidosis (hyperkalemic RTA)
• D Diarrhea-induced non-anion gap metabolic acidosis

Explanation

Type 1 (distal) RTA is characterized by inability of the distal tubule to secrete H+, leading to persistently alkaline urine pH (>5.5) even during systemic acidosis, hypokalemia (due to K-H exchange), hypercalciuria, nephrolithiasis (calcium phosphate stones, alkaline urine promotes precipitation), and nephrocalcinosis. Urinary anion gap (UAG = Na+K–Cl) is positive, indicating impaired NH4+ excretion. Type 2 RTA has urine pH <5.5 during acidosis and is characterized by bicarbonate wasting. Type 4 RTA presents with hyperkalemia.

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

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