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USMLE Step 1: Foundational Sciences Practice Exam 16

Practice exam for USMLE Step 1: Foundational Sciences.

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Question 1 of 2800%
Time remaining120:00
Attempts allowedUnlimited
Difficulty mixE • M • H
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Question 1

MEDIUM

A 67-year-old man presents to the emergency department with progressive shortness of breath and confusion for 1 day. He has severe COPD and uses home oxygen intermittently. He has smoked 1 pack of cigarettes daily for 45 years. Temperature is 37.1 C (98.8 F), blood pressure is 138/82 mm Hg, pulse is 96/min, and respiratory rate is 10/min. Arterial blood gas on room air shows: pH 7.28, PaCO2 68 mm Hg, PaO2 52 mm Hg, HCO3− 31 mEq/L. Which of the following best explains why administration of high-flow supplemental oxygen can worsen this patient's hypercapnia?

In chronic CO2 retainers with COPD, hypoxemia can contribute substantially to ventilatory drive. High-flow oxygen can blunt hypoxic drive, decreasing alveolar ventilation and increasing PaCO2. Additionally, oxygen can worsen V/Q mismatch by reversing hypoxic pulmonary vasoconstriction, but the best single explanation among choices is reduced hypoxic respiratory drive. Oxygen-induced pulmonary edema and increased metabolic CO2 production are not typical mechanisms here.

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