USMLE (United States Medical Licensing Examination) Practice Exam 03

A 26-year-old woman comes to the clinic for evaluation of episodic muscle weakness and palpitations for 3 months. She reports intense cravings for salty foods. She has no history of vomiting or diarrhea. Medications include an oral contraceptive. Blood pressure is 92/58 mm Hg. Physical examination shows mild orthostatic dizziness. Laboratory studies show: Na+ 129 mEq/L, K+ 5.8 mEq/L, Cl− 98 mEq/L, HCO3− 19 mEq/L, BUN 18 mg/dL, creatinine 0.9 mg/dL. Plasma renin activity is elevated, and serum aldosterone is low. Which of the following acid–base abnormalities is most likely present and what is the primary mechanism?