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USMLE Step 1: Foundational Sciences Practice Exam 18

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 woman presents to the emergency department with shortness of breath and lightheadedness for 6 hours. She returned yesterday from a 10-hour flight. She takes a combined oral contraceptive pill and has no other medical problems. Temperature is 37.0 C (98.6 F), blood pressure is 108/70 mm Hg, pulse is 112/min, respiratory rate is 24/min, and oxygen saturation is 92% on room air. Lungs are clear to auscultation. Arterial blood gas on room air shows: pH 7.49, PaCO2 30 mm Hg, PaO2 68 mm Hg. Which of the following best explains the cause of this patient's hypoxemia?

This presentation is consistent with pulmonary embolism (risk factors: long flight, OCPs) causing V/Q mismatch with increased dead space (ventilated but not perfused units), leading to hypoxemia and respiratory alkalosis from hyperventilation. Alveolar hypoventilation would raise PaCO2. Diffusion limitation is classically seen with interstitial fibrosis and worsens with exercise. A true right-to-left shunt is less responsive to oxygen and typically presents with different context (eg, congenital heart disease).

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