USMLE (United States Medical Licensing Examination): Step 1: Basic sciences assessment Practice Exam 10
Practice exam for USMLE (United States Medical Licensing Examination): Step 1: Basic sciences assessment.
Question 1
MEDIUMA 26-year-old man presents to the emergency department with shortness of breath and pleuritic chest pain for 6 hours. He returned yesterday from a 12-hour flight. He has no chronic medical conditions and takes no medications. Temperature is 37.2°C (99.0°F), blood pressure is 118/74 mm Hg, pulse is 112/min, respirations are 26/min, and oxygen saturation is 88% on room air. Arterial blood gas on room air shows pH 7.49, PaCO2 30 mm Hg, and PaO2 58 mm Hg. Which of the following best explains the increased alveolar-arterial (A–a) oxygen gradient in this patient?
This presentation is most consistent with pulmonary embolism after prolonged immobilization. PE causes areas of ventilation without perfusion (dead space), producing hypoxemia with an increased A–a gradient. Alveolar hypoventilation and high altitude typically cause hypoxemia with a normal A–a gradient. A shunt is perfusion without ventilation (eg, pneumonia, atelectasis), not the primary defect in PE.
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