Kaplan USMLE Step 1: Woman experiences marked hyperventilation

If you’re preparing for the United States Medical Licensing Examination® (USMLE®) Step 1 exam, you might want to know which questions are most often missed by test-prep takers. Check out this example from Kaplan Medical, and read an expert explanation of the answer. Also check out all posts in this series.

A 51-year-old woman is brought to the emergency department by her husband because of marked hyperventilation. Her temperature is 36.4°C (97.5°F), pulse is 122/min, respirations are 40/min and blood pressure is 145/92 mm Hg. She has a history of type 2 diabetes and hypertension for which she is currently taking medications, as well as chronic kidney disease. Laboratory studies show:

Creatinine: 2.4 mg/dL

pH: 7.22

Bicarbonate: 14 mEq/L

Sodium: 140 mEq/L

Potassium: 4.9 mEq/L

Chloride: 102 mEq/L

Which of the following drugs is most likely responsible for the findings in this patient? 

A. Atenolol 

B. Enalapril 

C. Furosemide 

D. Glyburide 

E. Metformin

The correct answer is E.

Metformin decreases gluconeogenesis and increases insulin sensitivity in type 2 diabetes. Metformin is renally cleared and accumulates in chronic kidney disease, and can precipitate lactic acidosis, although the mechanism behind this is poorly understood. The patient is hyperventilating as respiratory compensation for lactic acidosis.

Choice A. The patient is on unknown medications for type 2 diabetes and hypertension, has chronic kidney disease (CKD) and presents with metabolic acidosis. Metformin can precipitate lactic acidosis in the setting of renal insufficiency. Atenolol does not cause metabolic acidosis in the setting of CKD.