USMLE (United States Medical Licensing Examination) Practice Exam 18

A 24-year-old woman is brought to the emergency department after being found confused and breathing rapidly. She has a history of type 1 diabetes mellitus and reports 2 days of nausea and abdominal pain after running out of insulin. Temperature is 37.2°C (99.0°F), blood pressure is 98/60 mm Hg, pulse is 118/min, and respirations are 30/min with deep breathing. Labs show: Na+ 132 mEq/L, K+ 5.6 mEq/L, Cl− 96 mEq/L, HCO3− 10 mEq/L, glucose 520 mg/dL. Arterial blood gas shows pH 7.18, PaCO2 24 mm Hg. Which of the following is the most likely primary change that explains the low PaCO2?