Which of the following is a nonanion gap metabolic acidosis cause?

• A. Methanol
• B. Ureterostomy
• C. Lactic acidosis
• D. Ethylene glycol

Answer: (B)

Nonanion gap metabolic acidosis, also called hyperchloremic acidosis, happens when bicarbonate is lost or not reabsorbed while chloride rises to keep charge balance. The key idea is that the anion gap stays normal because there isn’t an accumulation of unmeasured acids.

Urinary diversion via ureterostomy is a classic cause because urine now passes through an intestinal segment. The intestinal mucosa reabsorbs chloride and ammonium and secretes bicarbonate into the urine. This shifts bicarbonate out of the body and chloride into the blood, producing a hyperchloremic, normal-anion-gap metabolic acidosis.

In contrast, methanol and ethylene glycol produce high-anion-gap acidosis because their metabolites (formate, glycolate) add unmeasured acids to the blood. Lactic acidosis is also a high-anion-gap process due to lactate accumulation.

So the ureterostomy stands out as the nonanion gap (hyperchloremic) metabolic acidosis cause.