USMLE (United States Medical Licensing Examination): Step 1: Basic sciences assessment Practice Exam 04
Practice exam for USMLE (United States Medical Licensing Examination): Step 1: Basic sciences assessment.
Question 1
MEDIUMA 67-year-old man presents to the emergency department with progressive shortness of breath and swelling of his legs for 2 weeks. He has a history of hypertension and prior myocardial infarction. Medications include lisinopril and furosemide. Temperature is 36.8°C (98.2°F), blood pressure is 148/92 mm Hg, pulse is 96/min, and respirations are 22/min. Examination shows bibasilar crackles and 2+ pitting edema to the knees. Laboratory studies show: sodium 132 mEq/L, potassium 4.6 mEq/L, creatinine 1.3 mg/dL. Urine sodium is 8 mEq/L (reference: variable). Which of the following best explains the low urine sodium concentration in this patient?
This patient has findings consistent with congestive heart failure with decreased effective arterial blood volume, which activates sympathetic tone and RAAS and increases proximal tubular sodium reabsorption, lowering urine sodium (prerenal physiology). A key distractor is inhibition of aldosterone: that would increase urine sodium. Decreased plasma oncotic pressure would reduce peritubular reabsorption and tend to increase natriuresis, not decrease it.
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