In a patient with normal anion gap metabolic acidosis, the urine anion gap (urine Na + urine K minus urine Cl) is strongly POSITIVE. This finding indicates:
- A Extrarenal loss of bicarbonate, as in diarrhea
- B Impaired renal ammonium excretion, as in distal RTA or renal failure ✓
- C Volume depletion with avid sodium reabsorption
- D Respiratory alkalosis with compensatory renal bicarbonate loss
Correct answer: B. Impaired renal ammonium excretion, as in distal RTA or renal failure
Explanation
The urine anion gap is a surrogate for urinary ammonium: NH4 is excreted with chloride, so high NH4 excretion makes urine chloride exceed Na plus K, giving a negative value as seen in diarrhea. A positive urine anion gap means ammonium excretion is inappropriately low, localizing the lesion to the kidney, as in distal RTA or advanced renal failure. Volume depletion does not determine this index.
Reference: Harrison's Principles of Internal Medicine, 21st ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.