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

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

Answer: (B)

Urine anion gap reflects how much ammonium (NH4+) the kidneys are excreting to counteract metabolic acidosis. In metabolic acidosis, the kidneys should boost NH4+ excretion, and this NH4+ is balanced in urine by accompanying chloride. When NH4+ excretion is high, chloride in urine rises and the urine anion gap tends to be negative. If NH4+ excretion is impaired, the chloride balance is reduced and the urine anion gap becomes positive.

Hyperkalemia points to reduced aldosterone action or resistance, which characterizes Type 4 RTA. In this form, diminished aldosterone effect lowers NH4+ production and excretion, giving a positive urine anion gap, while potassium tends to be elevated because of reduced distal potassium secretion.

Distal RTA can also show a positive urine anion gap, but it typically presents with hypokalemia rather than hyperkalemia. Proximal RTA usually preserves ammonium excretion and tends to have a negative urine anion gap with hypokalemia. Diuretic-induced acidosis can cause a positive gap, but the accompanying hyperkalemia specifically points toward aldosterone-related (Type 4) RTA.

So the combination of metabolic acidosis, a positive urine anion gap, and hyperkalemia is most consistent with Type 4 RTA.