USMLE (United States Medical Licensing Examination): Step 1: Basic sciences assessment Practice Exam 28
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 for 3 days. He has long-standing hypertension and type 2 diabetes mellitus. Medications include lisinopril and metformin. Temperature is 36.8°C (98.2°F), blood pressure is 172/96 mm Hg, pulse is 102/min, respirations are 24/min, and oxygen saturation is 90% on room air. Physical examination shows bibasilar crackles and 2+ pitting edema of the legs. Laboratory studies show:
BUN: 44 mg/dL (reference: 7-20) Creatinine: 2.2 mg/dL (reference: 0.6-1.3) Urine sodium: 8 mEq/L Urine osmolality: 520 mOsm/kg (reference: 50-1200)
He is diagnosed with acute decompensated heart failure with decreased renal perfusion. Which of the following changes is most likely present in the proximal tubule in response to this condition?
Decreased effective arterial blood volume (eg, heart failure) activates RAAS. Angiotensin II increases proximal tubular Na+ reabsorption (including via stimulation of the Na+/H+ exchanger), contributing to low urine sodium and concentrated urine. Increased NaCl delivery to the macula densa would occur with volume expansion or diuretic effect, not low perfusion. Pressure natriuresis increases urinary sodium with elevated renal perfusion pressure, which is not the case here.
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