A 26-year-old woman is evaluated for muscle weakness developing over the past several months. She has no focal symptoms and states that she otherwise feels well. Medical history is unremarkable, and there is no pertinent family history. She takes no medications.

On physical examination, blood pressure is 98/62 mm Hg, pulse rate is 98/min, and respiration rate is 16/min. BMI is 19. There is no lower extremity edema. The remainder of the examination is unremarkable.

Laboratory studies:

Serum electrolytes:

Sodium

142 mEq/L (142 mmol/L)

Potassium

3.1 mEq/L (3.1 mmol/L)

Chloride

120 mEq/L (120 mmol/L)

Bicarbonate

15 mEq/L (15 mmol/L)

Serum creatinine

1.2 mg/dL (106.1 µmol/L)

Urine electrolytes:

Sodium

18 mEq/L (18 mmol/L)

Potassium

8.0 mEq/L (8.0 mmol/L)

Chloride

32 mEq/L (32 mmol/L)

Urine pH

5.0

Urine dipstick

No blood or protein

Which of the following is the most likely cause of this patient's acid-base and electrolyte abnormalities?