USMLE (United States Medical Licensing Examination): Step 1: Basic sciences assessment Practice Exam 30
Practice exam for USMLE (United States Medical Licensing Examination): Step 1: Basic sciences assessment.
Question 1
MEDIUMA 68-year-old man presents to the emergency department with shortness of breath and productive cough for 3 days. He has chronic obstructive pulmonary disease and continues to smoke 1 pack of cigarettes daily. Temperature is 38.1°C (100.6°F), blood pressure is 138/84 mm Hg, pulse is 102/min, and respiratory rate is 26/min. He appears somnolent. Arterial blood gas on room air shows:
pH: 7.28 (reference: 7.35-7.45) PaCO2: 68 mm Hg (reference: 35-45 mm Hg) PaO2: 54 mm Hg (reference: 80-100 mm Hg) HCO3−: 31 mEq/L (reference: 22-26 mEq/L)
Which of the following best explains the elevated serum bicarbonate concentration in this patient?
This patient has acute-on-chronic respiratory acidosis (high PaCO2 with low pH) with renal compensation (elevated HCO3−). Chronic hypercapnia increases renal acid excretion, particularly via increased proximal tubule ammoniagenesis (glutamine metabolism) and increased H+ secretion, generating new bicarbonate that is returned to the blood. Carbonic anhydrase inhibition would lower serum HCO3− by causing bicarbonaturia. Aldosterone deficiency would tend to cause metabolic acidosis (type 4 RTA) rather than elevated bicarbonate.
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