A patient with distal renal tubular acidosis (Type 1 RTA) is expected to show which characteristic urine finding during a systemic metabolic acidosis?
- A Urine pH >5.5 (inappropriately alkaline) and positive urine anion gap ✓
- B Urine pH <5.5 and negative urine anion gap
- C Urine pH <5.5 and positive urine anion gap (impaired NH4+ excretion)
- D Urine pH >5.5 and negative urine anion gap (normal NH4+ excretion)
Explanation
In Type 1 (distal) RTA, the alpha-intercalated cells of the collecting duct fail to secrete H+ ions. As a result, urine cannot be acidified below pH 5.5 even in the presence of systemic acidosis (normal minimum urine pH should be <5.3). The urine anion gap (Na+ + K+ - Cl-) is positive, reflecting impaired NH4+ excretion (NH4+ is the major unmeasured cation in acidic urine). Type 4 RTA also has impaired NH4+ excretion (positive UAG) but shows hyperkalemia and normal urine acidification capacity. Type 2 (proximal) RTA can acidify urine below 5.5 when plasma bicarbonate falls below the reabsorption threshold.
Reference: Harrison's Principles of Internal Medicine, 21st ed.
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