USMLE (United States Medical Licensing Examination): Step 1: Basic sciences assessment Practice Exam 24
Practice exam for USMLE (United States Medical Licensing Examination): Step 1: Basic sciences assessment.
Question 1
MEDIUMA 62-year-old man presents to the emergency department with shortness of breath and progressive fatigue for 3 days. He has a history of chronic obstructive pulmonary disease and takes albuterol and tiotropium. He appears somnolent. Temperature is 37.0°C (98.6°F), blood pressure is 138/82 mm Hg, pulse is 96/min, respiratory rate is 10/min, and oxygen saturation is 86% on room air. Arterial blood gas analysis on room air shows:
pH: 7.28 PaCO2: 68 mm Hg PaO2: 52 mm Hg HCO3−: 31 mEq/L
Which of the following best explains the elevated bicarbonate level in this patient?
This patient has chronic hypercapnic respiratory acidosis with renal compensation (elevated HCO3−). The kidneys compensate by increasing net acid excretion, largely via increased ammoniagenesis in proximal tubule cells (glutamine metabolism) and increased H+ secretion, generating new bicarbonate. Decreased aldosterone would reduce H+ secretion and worsen acidosis rather than raise bicarbonate.
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