Acetaminophen overdose — MCCQE Part 1 MCQ

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HardPoisoning/OverdoseAcetaminophen overdoseMCCQE Part 1

A 27-year-old man becomes cyanotic and dyspnoeic after topical benzocaine. SpO2 remains 85% despite high-flow oxygen, arterial PaO2 is normal, and co-oximetry shows methemoglobin 28%. What is the most appropriate antidotal treatment?

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Correct answer: A — Methylene blue for acquired methemoglobinemia

Cyanosis with an oxygen-saturation plateau near 85%, normal dissolved arterial oxygen and elevated methemoglobin after benzocaine is acquired methemoglobinemia. Stop the oxidant, provide oxygen and give methylene blue for significant symptoms or a high level, in consultation with the poison centre. Co-oximetry, not standard pulse oximetry, establishes the abnormal hemoglobin fraction. Methylene blue can be ineffective or cause harm in important settings such as substantial G6PD deficiency and can interact with serotonergic drugs, so risks must be assessed. The other antidotes target opioid, cyanide, acetaminophen or sodium-channel toxicity.

Reference: Emergency Care BC, Common Toxidromes: https://emergencycarebc.ca/clinical_resource/clinical-summary/common-toxidromes-diagnosis-treatment/; Ontario Poison Centre, Emergency Antidotes 2026: https://www.ontariopoisoncentre.ca/health-care-providers/guidelines-for-stocking-emergency-antidotes-2026/