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USMLE (United States Medical Licensing Examination) Practice Exam 11

Practice exam for USMLE (United States Medical Licensing Examination).

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Question 1 of 1000%
Time remaining120:00
Attempts allowedUnlimited
Difficulty mixE • M • H
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Question 1

MEDIUM

A 24-year-old man is brought to the emergency department after being rescued from a house fire. He has soot around his nares and mouth and is confused. Pulse oximetry reads 99% on a nonrebreather mask. Arterial blood gas shows: pH 7.31, PaCO2 32 mm Hg, PaO2 310 mm Hg. Co-oximetry shows elevated carboxyhemoglobin level. Which of the following is the most likely explanation for the discrepancy between the pulse oximetry reading and the patient’s clinical condition?

Pulse oximeters cannot distinguish carboxyhemoglobin from oxyhemoglobin well, leading to falsely normal/high SpO2 despite impaired oxygen content and delivery. PaO2 can be normal/high because it reflects dissolved oxygen, not hemoglobin-bound oxygen; pulse oximetry does not directly measure PaO2.

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