USMLE (United States Medical Licensing Examination): Step 1: Basic sciences assessment Practice Exam 26
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 confusion for 1 day. He has severe COPD and uses home oxygen intermittently. He has been increasingly somnolent since starting a new sedating cough suppressant. Temperature is 37.0°C (98.6°F), blood pressure is 138/82 mm Hg, pulse is 92/min, respiratory rate is 8/min, and oxygen saturation is 86% on room air. Arterial blood gas shows:
pH: 7.25 PaCO2: 70 mm Hg PaO2: 55 mm Hg HCO3−: 30 mEq/L
Which of the following best explains the change in his arterial pH?
This patient has acute-on-chronic respiratory acidosis (high PaCO2, low pH) with partially elevated HCO3− from renal compensation. The kidney compensates for chronic hypercapnia by increasing net acid excretion, largely via increased ammoniagenesis (NH3) in proximal tubule cells and increased H+ secretion, raising plasma HCO3−.
Alpha-intercalated cell H+ secretion increases (not decreases). Increased ventilation would lower PaCO2 and raise pH, opposite of the scenario.
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