In distal renal tubular acidosis (Type 1 RTA), the defect is failure of the collecting duct to secrete H+ ions. The biochemical consequence that distinguishes Type 1 from Type 2 RTA on urine pH measurement is:
- A Type 1 RTA has urine pH <5.5 because proximal HCO3- reclamation is normal; Type 2 has pH >5.5 due to bicarbonate wasting
- B Both types produce urine pH >5.5 in normal conditions but Type 1 uniquely causes hypokalemia while Type 2 causes hyperkalemia
- C Type 1 RTA cannot acidify urine below pH 5.5 even during systemic acidosis; Type 2 RTA can acidify urine to <5.5 once plasma HCO3- falls below the lowered proximal threshold ✓
- D Type 1 RTA causes urine pH >7.0 (alkaline urine) due to bicarbonate secretion
Explanation
The key distinguishing test is minimum urine pH during systemic acidosis. In Type 1 (distal) RTA, the alpha-intercalated cells of the collecting duct lack functional H+-ATPase (or Cl-/HCO3- exchanger), preventing acid secretion even when plasma pH is very low—urine pH remains >5.5 always. In Type 2 (proximal) RTA, the proximal tubule has reduced HCO3- reabsorption threshold (lowered Tm for bicarbonate). When plasma HCO3- falls below the defective threshold, the distal nephron functions normally and can acidify urine to <5.5. Both types cause hypokalemia (not hyperkalemia), and Type 4 RTA causes hyperkalemia. Urine pH >7.0 specifically occurs when urease-producing bacteria alkalinise urine, not as a feature of RTA.
Reference: Harper's Illustrated Biochemistry, 32nd ed.
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