Which of the following is a common cause of anion gap metabolic acidosis?

• A. Ureterostomy
• B. Adrenal insufficiency
• C. Pancreatic fistula
• D. Methanol

Answer: (D)

Anion gap metabolic acidosis happens when acids introduced into the body add unmeasured anions, so the calculated gap Na minus (Cl plus HCO3) rises. These unmeasured anions include things like formate, lactate, and ketoacids. Methanol is a classic toxin that causes this pattern because it is converted in the liver to formaldehyde and then to formic acid. Formate accumulates as an unmeasured anion, driving a high anion gap and lowering bicarbonate, which is the hallmark of this type of acidosis. Early in methanol poisoning you may also see an osmolar gap from unmetabolized methanol, which helps with recognition. Treatment focuses on blocking the toxic metabolism with fomepizole (or ethanol), correcting the acidosis, and removing the toxin with dialysis if severe.

The other options tend to produce non-anion gap (hyperchloremic) acidosis rather than an anion gap type. For example, bicarbonate loss through a urinary diversion or pancreatic fistula lowers bicarbonate and elevates chloride, but keeps the anion gap normal. Adrenal insufficiency can cause metabolic disturbances, often involving lactic acidosis as a component, but it’s not the classic toxin-related cause of a high anion gap. In that sense, methanol stands out as a common cause of anion gap metabolic acidosis.