USMLE (United States Medical Licensing Examination): Step 1: Basic sciences assessment Practice Exam 01
Practice exam for USMLE (United States Medical Licensing Examination): Step 1: Basic sciences assessment.
Question 1
MEDIUMA 26-year-old woman presents to the emergency department with shortness of breath and lightheadedness for 6 hours. She delivered a baby 5 days ago and has had heavy vaginal bleeding since delivery. She takes no medications. Temperature is 36.8°C (98.2°F), blood pressure is 84/52 mm Hg, pulse is 128/min, and respiratory rate is 24/min. Physical examination shows cool extremities and delayed capillary refill. Laboratory studies show hemoglobin 7.1 g/dL. After rapid infusion of isotonic saline, her blood pressure improves to 102/64 mm Hg. Which of the following changes is most likely to occur in the afferent arteriole of this patient’s kidneys during the initial hypotensive period (before fluids are given)?
Acute hemorrhage causes decreased effective arterial blood volume, activating sympathetic tone and RAAS. Angiotensin II preferentially constricts the efferent arteriole, but sympathetic activation also causes afferent arteriolar constriction to preserve systemic pressure, reducing RBF and GFR. Prostaglandins tend to dilate the afferent arteriole and counteract excessive vasoconstriction, but the dominant initial response in shock is vasoconstriction. Autoregulation fails when perfusion pressure drops below the autoregulatory range.
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