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USMLE Step 1: Foundational Sciences Practice Exam 17

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 27-year-old man presents to the emergency department with shortness of breath and lightheadedness for 2 hours after a long-distance flight. He has no past medical history and takes no medications. Temperature is 37.0 C (98.6 F), blood pressure is 118/72 mm Hg, pulse is 112/min, respiratory rate is 26/min, and oxygen saturation is 90% on room air. He appears anxious and is tachypneic. Arterial blood gas on room air shows: pH 7.49, PaCO2 30 mm Hg, PaO2 62 mm Hg. Which of the following best explains the mechanism responsible for this patient's decreased arterial oxygenation?

This presentation is consistent with acute pulmonary embolism after prolonged immobility. PE causes areas of ventilation without perfusion (high V/Q), increasing alveolar dead space and producing V/Q mismatch with hypoxemia and respiratory alkalosis from hyperventilation. A right-to-left shunt classically does not correct with oxygen; PE typically improves with supplemental oxygen because some units remain perfused. Diffusion limitation is more typical of interstitial fibrosis and worsens with exercise.

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