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Carbon monoxide poisoning — USMLE Step 3 MCQ

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ModerateEmergency MedicineCarbon monoxide poisoningUSMLE Step 3

A family develops headache, nausea, dizziness, and confusion after using a generator indoors. Pulse oximetry reads 98% on room air, but carboxyhemoglobin is elevated. Which of the following is the most likely explanation for this patient's findings?

Educational content. Not a substitute for clinical judgement or local policy.

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Correct answer: C — Pulse oximetry cannot distinguish oxyhemoglobin from carboxyhemoglobin

Standard pulse oximetry may appear normal in carbon monoxide poisoning because it misreads carboxyhemoglobin as oxyhemoglobin.

Reference: CDC carbon monoxide poisoning guidance