Syndrome of inappropriate antidiuretic hormone secretion Quiz: Ace Your Exams

A 52-year-old woman presents to the clinic with complaints of fatigue, headaches, and nausea over the past week. She has no fevers, chest pain, or shortness of breath. She has a past medical history of congestive heart failure, for which she takes furosemide, and major depressive disorder, for which she started sertraline 3 weeks ago. She does not use tobacco. She occasionally drinks alcohol. Her temperature is 36.5°C (97.7°F), heart rate is 78/min, blood pressure is 120/75 mm Hg, respiratory rate is 16/min, and oxygen saturation is 99% on room air. On physical examination, she appears tired but is fully oriented. Mucous membranes are moist. Cardiac auscultation reveals a regular rhythm without murmurs, rubs, or gallops. Pulmonary examination is within normal limits, and the abdomen is soft and nontender. No peripheral edema or jugular venous distension is seen. Laboratory evaluation is significant for a serum sodium of 125 mEq/L (reference range: 136-146 mEq/L). Which of the following is the most likely diagnosis?