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USMLE (United States Medical Licensing Examination) Practice Exam 07

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

120:00

A 26-year-old man is brought to the emergency department after being found confused and breathing rapidly after a day of profuse diarrhea. He has no significant medical history. Temperature is 37.0°C (98.6°F), blood pressure is 98/60 mm Hg, pulse is 118/min, and respirations are 32/min. Mucous membranes are dry. Arterial blood gas shows pH 7.28, PaCO2 28 mm Hg, and HCO3− 13 mEq/L. Which of the following is the most likely physiologic mechanism responsible for the low PaCO2?

Metabolic acidosis from diarrhea (low HCO3−) triggers compensatory hyperventilation primarily via peripheral chemoreceptors (carotid bodies) sensing low arterial pH, lowering PaCO2. Central chemoreceptors respond mainly to CO2/pH in CSF and are less directly responsive to metabolic acidosis.

Select one answer

Decreased central chemoreceptor stimulation by cerebrospinal fluid pH

Decreased pulmonary capillary blood flow causing increased dead space

Increased peripheral chemoreceptor firing due to low arterial pH

Reduced alveolar ventilation due to respiratory muscle fatigue

Rightward shift of the oxyhemoglobin dissociation curve increasing oxygen unloading

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USMLE (United States Medical Licensing Examination) Practice Exam 07

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