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USMLE (United States Medical Licensing Examination): Step 1: Basic sciences assessment Practice Exam 06

Practice exam for USMLE (United States Medical Licensing Examination): Step 1: Basic sciences assessment.

Active sessionUSMLE Step 1 practice testUSMLE Step 1 practice examUSMLE Step 1 practice questionsUSMLE Step 1 basic sciencesUSMLE Step 1 foundational sciences+5
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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 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. Medications include albuterol and tiotropium. He is somnolent. Temperature is 37.1°C (98.8°F), blood pressure is 146/84 mm Hg, pulse is 96/min, respirations are 10/min, and oxygen saturation is 88% on room air. Arterial blood gas shows: pH 7.28, PaCO2 68 mm Hg, PaO2 54 mm Hg, HCO3− 31 mEq/L. Which of the following best explains the mechanism of hypoxemia in this patient?

This patient has acute-on-chronic hypercapnic respiratory failure with hypoventilation (low respiratory rate, elevated PaCO2). By the alveolar gas equation, increased PaCO2 lowers alveolar oxygen tension (PAO2), leading to hypoxemia. V/Q mismatch is common in COPD but the vignette emphasizes hypoventilation with marked hypercapnia and depressed respirations. An anatomic shunt typically causes hypoxemia that is refractory to supplemental oxygen; diffusion limitation is classically exertional and associated with interstitial disease.

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