Review Question - QID 213731

A 13-year-old boy presents to the emergency department with a fever and increased urination for several days. He states that he has had a cough and runny nose during this time as well as a severe headache. He endorses urinating up to 20 times per day and states that he fainted and hit his head today. He says he otherwise feels weak and has not been eating or taking his medications as a result. The patient cannot recall his past medical history nor his medications. His temperature is 101°F (38.3°C), blood pressure is 93/67 mmHg, pulse is 166/min, respirations are 33/min, and oxygen saturation is 100% on room air. Physical exam is notable for a fatigued appearing boy with dry mucous membranes and irritability. Initial laboratory studies are ordered as seen below.

Hemoglobin: 12 g/dL
Hematocrit: 36%
Leukocyte count: 19,900/mm^3 with normal differential
Platelet count: 199,000/mm^3

Serum:
Na+: 139 mEq/L
Cl-: 101 mEq/L
K+: 2.5 mEq/L
HCO3-: 6.0 mEq/L
BUN: 23 mg/dL
Glucose: 399 mg/dL
Creatinine: 1.2 mg/dL
Ca2+: 10.2 mg/dL

The patient is started on normal saline with potassium as well as insulin once his potassium is 3.2 mEq/L. Over the next 3 hours, the patient becomes progressively more confused and is noted to have urinated in the bed. He subsequently demonstrates tonic-clonic motions. Which of the following is the most likely etiology of this patient's most recent symptoms?