A 38-year-old man is evaluated in the emergency department for acute abdominal pain. Medical history is significant for excessive alcohol use and recurrent acute pancreatitis. He drinks six beers daily. He takes no medications.

On physical examination, the patient is in acute distress and indicates epigastric pain. Temperature is 38.0 °C (100.4 °F), blood pressure is 160/88 mm Hg, pulse rate is 88/min, and respiration rate is 20/min. BMI is 25. Chest and heart examinations are normal. The abdomen is slightly distended, with tenderness to minimal palpation in the epigastric area. There is no peripheral edema.

Laboratory studies:

Leukocyte count

10,000/µL (10 × 109/L)

Blood urea nitrogen

15 mg/dL (5.4 mmol/L)

Creatinine

1.2 mg/dL (106.1 µmol/L)

Electrolytes:

Sodium

128 mEq/L (128 mmol/L)

Potassium

4.0 mEq/L (4.0 mmol/L)

Chloride

99 mEq/L (99 mmol/L)

Bicarbonate

24 mEq/L (24 mmol/L)

Glucose

90 mg/dL (5 mmol/L)

Lipase

620 U/L

Plasma osmolality

290 mOsm/kg H2O

Urine osmolality

400 mOsm/kg H2O

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