You are answering a USMLE-style medical question. Question: A 75-year-old man comes to the physician because of a 7-day history of nausea and vomiting. Over the past 2 days, he has also been feeling weak and tired. When standing up after sitting for a while, he feels dizzy. He says he has to go to the bathroom more often than usual, and that he is urinating “a normal amount” each time. He has not had diarrhea. He has hypertension, for which he has been taking hydrochlorothiazide for the past 6 months. He drinks 9 glasses of water per day and takes his medication regularly. He is 168 cm (5 ft 6 in) tall and weighs 90 kg (198 lb); BMI is 32 kg/m2. His temperature is 36.5°C (97.7°F), blood pressure is 106/54 mm Hg, and pulse is 92/min. Physical examination shows whitening of the tongue. Skin that is pinched on the back of the hand retracts after 5 seconds. On mental status examination, his speech is slowed; he is oriented to person, place, and time. Laboratory studies show: Serum Na+ 150 mEq/L Cl− 97 mEq/L K+ 3.6 mEq/L HCO3− 30 mEq/L Osmolality 354 mOsm/kg Hemoglobin A1C 10.5% Urine Osmolality 400 mOsm/kg Which of the following is the most likely explanation for these findings?" Options: A) Diuretic overdose B) Osmotic diuresis C) Excess production of aldosterone D) Insufficient production of antidiuretic hormone Reply with ONLY the letter of the correct answer (A, B, C, or D). No explanation.