A 34-year-old woman is evaluated for laboratory abnormalities discovered as part of an evaluation for joint pain. She describes the pain as being diffuse and associated with chronically dry eyes and mouth. She also notes recent-onset mild nocturia. Medical history is otherwise negative, and she takes acetaminophen daily for her joint pain.

On physical examination, vital signs are normal. BMI is 23. Mucous membranes and conjunctivae are dry, and mild parotid enlargement is present. There is no evidence of joint inflammation. The remainder of the examination is unremarkable.

Laboratory studies:

Creatinine

0.9 mg/dL (79.6 µmol/L)

Electrolytes:

Sodium

138 mEq/L (138 mmol/L)

Potassium

3.1 mEq/L (3.1 mmol/L)

Chloride

118 mEq/L (118 mmol/L)

Bicarbonate

12 mEq/L (12 mmol/L)

Glucose

74 mg/dL (4.1 mmol/L)

Urinalysis

pH 7.0; no blood, glucose, protein, erythrocytes, or leukocytes

Which of the following is the most likely cause of this patient's laboratory findings?