In type 1 renal tubular acidosis (distal RTA), the underlying defect is failure of H+ secretion in the collecting duct. The characteristic biochemical finding that distinguishes it from other causes of non-anion-gap metabolic acidosis is:

• A Urine pH < 5.5 despite systemic acidosis
• B Urine pH > 5.5 despite systemic acidosis, with hypokalemia and hyperchloremia ✓
• C Elevated anion gap due to accumulation of beta-hydroxybutyrate
• D Normal urine pH with hyperkalemia and urinary potassium wasting

Explanation

In type 1 (distal) RTA, the alpha-intercalated cells of the collecting duct cannot acidify urine maximally due to defective H+-ATPase or anion exchanger AE1 (SLC4A1). Even in the presence of severe systemic acidosis, urine pH cannot fall below 5.5 (classic finding: urine pH persistently > 5.5 or ≥ 6.0). This is accompanied by hypokalemia (K+ wasting as a trade-off for Na+ reabsorption without H+ secretion), hyperchloremia, and nephrocalcinosis/nephrolithiasis (due to alkaline urine precipitating calcium phosphate). Option A describes a normal acidifying response or type 2 (proximal) RTA. Type 4 RTA has hyperkalemia.

Reference: Harper's Illustrated Biochemistry, 32nd ed.

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