A 28-year-old woman has plasma pH 7.31, bicarbonate 14 mEq/L, pCO2 30 mmHg, serum Na 138 mEq/L, Cl 110 mEq/L, K 2.8 mEq/L. Anion gap = 14 mEq/L (normal). Urine pH is 6.2 (alkaline). Urine anion gap is +10 (positive). Which type of RTA is present?

• A Type 2 (proximal) RTA — defective proximal HCO3 reabsorption; urine pH variable
• B Type 4 RTA — hypoaldosteronism; associated with hyperkalaemia
• C High anion gap metabolic acidosis with respiratory compensation
• D Type 1 (distal) RTA — defective proton secretion in collecting duct; urine pH cannot fall below 5.3 ✓

Explanation

This is Type 1 (distal) RTA: normal anion gap metabolic acidosis (AG = 138 - 110 - 14 = 14, normal ≤ 12 — slight elevation due to low bicarb formula; effectively normal AG), alkaline urine pH (> 5.3, cannot acidify below this in Type 1), hypokalaemia, and positive urine anion gap (UAG = uNa + uK - uCl > 0, indicating low urinary NH4+ and impaired ammonium excretion). Type 2 (proximal) RTA has urine pH that eventually falls below 5.3 once serum HCO3 drops below the proximal tubular threshold. Type 4 causes hyperkalaemia from aldosterone deficiency/resistance.

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

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