Review Question - QID 108993

A 69-year-old male presents to the emergency department for sudden shortness of breath that came on while he was sleeping. He currently is short of breath but denies any chest pain, arm pain, or jaw pain. The patient has a past medical history of hypertension, obesity, diabetes, and GERD. The patient states he has not filled his prescriptions for any of his medications in months and cannot remember what his medications are. An EKG is performed and is seen in Figure A. A chest radiograph is obtained and is seen in Figure B. Lab values at admission are below:

Serum:
Na+: 139 mEq/L
K+: 4.0 mEq/L
Cl-: 100 mEq/L
HCO3-: 24 mEq/L
BUN: 22 mg/dL
Glucose: 72 mg/dL
Ca2+: 9.9 mg/dL
Mg2+: 1.5 mEq/L
Creatinine: 0.9 mg/dL

The patient is started on appropriate treatment and his symptoms resolve. He is recovering on the medicine floor. His laboratory values are repeated and are shown below:

Serum:
Na+: 137 mEq/L
K+: 3.5 mEq/L
Cl-: 100 mEq/L
HCO3-: 26 mEq/L
BUN: 39 mg/dL
Glucose: 70 mg/dL
Ca2+: 9.7 mg/dL
Mg2+: 1.5 mEq/L
Creatinine: 1.5 mg/dL

The patient states that he has not urinated in the past 24 hours. Which of the following is the most likely cause of this patient's current presentation?