If you’re preparing for the United States Medical Licensing Examination® (USMLE®) Step 3 exam, you might want to know which questions are most often missed by test-prep takers. Check out this example from Kaplan Medical, and read an expert explanation of the answer. Also check out all posts in this series.  

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You are called to see a 64-year-old woman who has been complaining of progressive shortness of breath over the past two days. She was admitted to the hospital five days ago after a fall and has been on bed rest for a nondisplaced pubic ramus fracture. She has been on deep vein thrombosis prophylaxis with subcutaneous heparin. Her medical history is significant for type 2 diabetes and dialysis-dependent renal failure secondary to diabetic nephropathy. She is anuric at baseline.

The patient’s last dialysis run was four days ago due to a scheduling error in the hospital. Her blood pressure is 180/100 mmHg, pulse is 100 beats per minute, respirations are 22 per minute, and oxygen saturation is 85% on room air. She has a jugular venous pressure (JVP) of 10 cm and inspiratory crackles halfway up from the bases upon auscultation of the lungs. An electrocardiogram (ECG) shows a rate-related right bundle branch block but no ischemic changes. Chest x-ray reveals interstitial edema and vascular redistribution to the apices.

After administering oxygen to achieve an oxygen saturation greater than 93% and nitrates to control her blood pressure, the most appropriate next therapeutic intervention is to:

A. Administer intravenous furosemide.

B. Administer lysis therapy.

C. Arrange for emergent hemodialysis.

D. Arrange for urgent ventilation perfusion scan.

E. Begin intravenous heparin.

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The correct answer is C.

The patient is most likely to be in pulmonary edema from volume-overload congestive heart failure, most likely from a missed dialysis run. Most people who are anuric will have at least three dialysis runs per week, and this patient had not had a dialysis run in four days. The clues to her volume overload include an elevated JVP, hypertension, crackles on lung exam, and pulmonary edema on the chest x-ray.

Choice A: Because she does not make urine (stated in the question), the patient is unlikely to respond to IV diuretics. Instead, the patient needs emergent hemodialysis to remove volume.

Choice B, D and E: Arrange for urgent ventilation perfusion scan, administering lysis therapy, or beginning intravenous heparin for presumed pulmonary embolus are diagnostic and therapeutic interventions for pulmonary embolism (PE). While the patient's immobility certainly places her at greater risk for DVT and pulmonary embolism, she has been on DVT prophylaxis with SQ heparin.

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Also, PE tends to cause acute dyspnea rather than progressive symptoms and may cause pleuritic chest pain and findings of right heart strain on the ECG. Classically, PE does not present as pulmonary edema on the chest x-ray. While PE certainly is difficult to rule out short of performing a pulmonary angiogram in this scenario, the overwhelming evidence suggests congestive heart failure from volume overload.

For more prep questions on USMLE Steps 1, 2 and 3, view  other posts in this series.

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