A 30-year-old man found unconscious has severe high anion gap metabolic acidosis. Serum electrolytes: Na+ 140, Cl- 105, HCO3- 8 mEq/L. Measured serum osmolality is 340 mOsm/kg while calculated osmolality (2Na + glucose/18 + BUN/2.8) is 295 mOsm/kg. Urine microscopy shows envelope-shaped crystals. Which toxin best explains this picture?

• A Methanol
• B Salicylates
• C Isopropyl alcohol
• D Ethylene glycol ✓

Explanation

Ethylene glycol is metabolised by alcohol dehydrogenase to glycolic acid, causing high anion gap acidosis, while the unmetabolised parent alcohol raises the measured osmolality producing a large osmolal gap. Its metabolite oxalic acid binds calcium, forming calcium oxalate monohydrate or dihydrate (envelope-shaped) crystals in urine. Methanol causes formic acid accumulation with visual symptoms but no oxalate crystals, and isopropyl alcohol causes ketosis without acidosis.

Reference: Harrison's Principles of Internal Medicine, 21st ed.

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Written and medically reviewed by the StethoPrep medical team.