In a patient with metabolic acidosis and a negative urine anion gap, which is most likely the cause?

• A. Renal tubular acidosis type 1
• B. Diarrhea (excreting NH4)
• C. Renal failure
• D. Urinary tract infection

Answer: (B)

The main idea is how the urine anion gap reflects the kidney’s handling of acid in metabolic acidosis. The urine anion gap is calculated as Na+ plus K+ minus Cl− in the urine. When the body is in metabolic acidosis, the kidneys compensate by increasing ammonium (NH4+) production and excretion. Ammonium excretion is coupled with chloride excretion (as NH4Cl) to maintain electroneutrality, so a lot of chloride is dumped in the urine. That extra chloride makes the urine anion gap negative.

A negative urine anion gap therefore points to an intact renal ability to excrete ammonium, which is what happens when the acidosis is due to loss of bicarbonate from the gut, such as in diarrhea. The kidneys respond by increasing NH4+ (and Cl−) excretion, yielding a negative UAG. This pattern helps distinguish extrarenal bicarbonate losses from renal causes.

In renal causes of metabolic acidosis (like distal renal tubular acidosis or renal failure), NH4+ excretion is impaired, so the urine anion gap tends to be positive or not negative. A urinary tract infection doesn’t fit the classic pattern for a negative UAG in this context, so it’s not the best explanation for a metabolic acidosis with a negative UAG.

So diarrhea, by causing bicarbonate loss from the gut and prompting renal NH4+ excretion with accompanying chloride, best explains a negative urine anion gap.