A patient after ingesting methanol develops severe metabolic acidosis with high anion gap, visual disturbances, and 'snowstorm vision'. The toxic metabolite responsible for optic nerve toxicity is:

• A Formaldehyde (methanal)
• B Acetaldehyde from partial oxidation by CYP2E1
• C Oxalic acid from co-ingested ethylene glycol
• D Formic acid (formate ion at physiological pH) ✓

Explanation

Methanol is metabolised by alcohol dehydrogenase first to formaldehyde and then to formic acid (formate). While formaldehyde is also toxic, it is rapidly converted to formate. Formate (formic acid at physiological pH) is the principal toxic metabolite responsible for the specific optic nerve toxicity of methanol — it inhibits cytochrome c oxidase in optic nerve mitochondria, causing oxidative phosphorylation failure in retinal ganglion cells and the optic nerve, leading to 'snowstorm vision', papilloedema, and permanent blindness. The treatment with ethanol or fomepizole blocks alcohol dehydrogenase to prevent formate accumulation.

Reference: The Essentials of Forensic Medicine and Toxicology (Narayan Reddy), 34th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.