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USMLE (United States Medical Licensing Examination) Practice Exam 21

Practice exam for USMLE (United States Medical Licensing Examination).

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Question 1 of 1000%
Time remaining120:00
Attempts allowedUnlimited
Difficulty mixE • M • H
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Question 1

MEDIUM

A 67-year-old man with long-standing chronic obstructive pulmonary disease presents for routine follow-up. He uses albuterol and tiotropium. He has chronic dyspnea on exertion but no acute change. Pulse oximetry is 91% on room air. Arterial blood gas shows pH 7.36, PaCO2 58 mm Hg, PaO2 62 mm Hg, HCO3− 32 mEq/L. Which of the following is the most likely explanation for this patient’s elevated serum bicarbonate level?

Chronic respiratory acidosis from COPD is compensated by the kidneys via increased H+ secretion (including ammonium trapping) and generation/reabsorption of HCO3−. Carbonic anhydrase inhibition would decrease HCO3− reabsorption and cause metabolic acidosis; vomiting causes metabolic alkalosis but would not fit the chronic hypercapnic ABG pattern.

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