USMLE (United States Medical Licensing Examination): Step 1: Basic sciences assessment Practice Exam 33
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 shortness of breath and confusion for 1 day. He has severe COPD and uses home oxygen. Medications include tiotropium and albuterol. Temperature is 37.1°C (98.8°F), blood pressure is 146/84 mm Hg, pulse is 96/min, and respiratory rate is 10/min. Pulse oximetry shows 92% on 4 L/min nasal cannula. Arterial blood gas shows: pH 7.28, PaCO2 68 mm Hg, PaO2 62 mm Hg, HCO3− 31 mEq/L. Which of the following best explains the elevated serum bicarbonate concentration in this patient?
Chronic hypercapnia (COPD) leads to a respiratory acidosis; renal compensation occurs over days via increased proximal tubule ammoniagenesis and increased net acid excretion, generating “new” bicarbonate and raising plasma HCO3−. Decreased aldosterone would reduce H+ secretion. Intercalated cells in compensation increase H+ secretion and HCO3− reabsorption, not bicarbonate secretion. Contraction alkalosis is related to volume depletion. Respiratory alkalosis would lower PaCO2 and is inconsistent with the ABG.
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