A patient with metabolic acidosis has a positive urine anion gap and hypokalemia. This pattern most strongly suggests which RTA?

• A. Type 4 RTA
• B. Proximal RTA
• C. Distal/type 1 RTA
• D. Diuretic-induced acidosis

Answer: (C)

The key idea is how the kidneys excrete acid in metabolic acidosis and what the urine anion gap tells us about ammonium (NH4+) excretion. In acidosis, a healthy kidney increases NH4+ excretion to neutralize acid, and this NH4+ is accompanied by Cl−, so the urine tends to have a negative anion gap. A positive urine anion gap means NH4+ excretion is not adequately upregulated, signaling impaired distal acidification of the nephron.

Distal RTA arises from a defect in hydrogen ion secretion in the collecting duct, so the urine cannot be properly acidified. This reduces NH4+ production/excretion and makes the urine anion gap positive. The accompanying hypokalemia fits distal RTA, since potassium loss is common when the distal nephron is dysfunctional. In contrast, Type 4 RTA typically presents with hyperkalemia, not hypokalemia, and proximal (Type 2) RTA usually preserves distal acidification and often shows a negative UAG. Diuretic-induced acidosis isn’t a primary RTA pattern and doesn’t define this ammonium-handling defect.