Electrolyte Disorders USMLE Step 1 Practice Question

A 56-year-old man with a 5-year history of hypertension has been taking hydrochlorothiazide 25 mg daily for the past 3 months. He presents to the clinic with a 2-week history of progressive muscle cramps, generalized weakness, and mild confusion. His wife reports he has been more lethargic than usual. Vital signs: BP 132/78 mmHg, HR 108/min, RR 16/min, temperature 37.0°C. Orthostatic blood pressure drop of 12 mmHg is noted. Laboratory studies: Serum: Na 127 mEq/L, K 2.8 mEq/L, Cl 94 mEq/L, HCO3 32 mEq/L, BUN 28 mg/dL, Cr 1.3 mg/dL Osmolality: serum 283 mOsm/kg, urine 485 mOsm/kg Urine Na 15 mEq/L, Cl 12 mEq/L Physical exam: patient appears volume-depleted with dry mucous membranes and decreased skin turgor. Which of the following best explains the development of hyponatremia in this patient?