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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.

Active sessionUSMLE Step 1 practice testUSMLE Step 1 practice examUSMLE Step 1 practice questionsUSMLE Step 1 basic sciencesUSMLE Step 1 foundational sciences+5
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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 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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