This patient has an increased anion gap metabolic acidosis, and the most likely cause is D-lactic acidosis. Accumulation of the D-isomer of lactate can occur in patients with short-bowel syndrome following jejunoileal bypass or small-bowel resection. In these patients, excess carbohydrates that reach the colon are metabolized to D-lactate. Symptoms include intermittent confusion, slurred speech, and ataxia. Laboratory studies show increased anion gap metabolic acidosis with normal plasma lactate levels, because the D-isomer is not measured by conventional laboratory assays for lactate. Diagnosis should be considered in patients with an unexplained increased anion gap metabolic acidosis in the appropriate clinical context; it is confirmed by specifically measuring D-lactate.

Ethylene glycol or methanol intoxication should be suspected in patients with an increased anion gap metabolic acidosis associated with a serum bicarbonate level <10 mEq/L (10 mmol/L) and a plasma osmolal gap >10 mOsm/kg H₂O. This patient's serum bicarbonate level is 20 mEq/L (20 mmol/L) and the calculated osmolal gap is 6 mOsm/kg H₂O, making alcohol poisoning a less likely diagnosis.

Propylene glycol is a solvent used as a vehicle for numerous intravenously administered medications. In propylene glycol toxicity, laboratory findings include increased anion gap metabolic acidosis and concomitant increased osmolal gap. The metabolic acidosis is principally due to L-lactic and D-lactic acidosis, the acid metabolites of propylene glycol. This patient's clinical history and osmolal gap are not consistent with propylene glycol toxicity.

Pyroglutamic acidosis is a cause of increased anion gap metabolic acidosis in patients receiving therapeutic doses of acetaminophen on a chronic basis. Clinical manifestations are limited to mental status changes and increased anion gap metabolic acidosis. The syndrome most commonly occurs in patients with critical illness, poor nutrition, liver disease, or chronic kidney disease as well as in persons on a vegetarian diet. Diagnosis can be confirmed by measuring urine levels of pyroglutamic acid. This patient's history is most compatible with D-lactic acidosis and does not support acetaminophen ingestion.