A 32-year-old woman with a history of Sjögren syndrome presents with fatigue and polyuria. Laboratory studies reveal a normal anion gap metabolic acidosis with serum bicarbonate of 15 mEq/L and potassium of 4.2 mEq/L. Her urine pH is persistently 6.5 despite systemic acidosis, and urinary ammonium excretion is low. Which of the following best explains her acid–base disturbance?
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