Review Question - QID 213825

A 74-year-old man presents to the emergency department with a change in his behavior. Over the past few days the patient’s wife noticed he has become increasingly lethargic. The patient was found sleeping in his recliner soaked in urine. He has a past medical history of diabetes, a 20 pack-year smoking history, and hypertension; however, since moving 3 years ago he has not had time to follow up with a new primary care doctor and establish care. The only medications he has continued to take are his hydrochlorothiazide and lisinopril from leftover prescriptions. His temperature is 97.0°F (36.1°C), blood pressure is 104/54 mmHg, pulse is 138/min, respirations are 14/min, and oxygen saturation is 98% on room air. Physical exam is notable for a confused and elderly patient with midline lower spinal tenderness. He has dry mucous membranes. The patient's hemoglobin is 9.0 g/dL. Additional laboratory values are ordered as seen below.

Serum:
Na+: 139 mEq/L
Cl-: 101 mEq/L
K+: 3.9 mEq/L
HCO3-: 28 mEq/L
BUN: 40 mg/dL
Glucose: 99 mg/dL
Creatinine: 1.8 mg/dL
Ca2+: 13.7 mg/dL

Which of the following is the most likely etiology of this patient’s symptoms?