Review Question - QID 214738

A 58-year-old man with a history of alcohol use disorder, type 2 diabetes, benign essential tremor, intravenous drug use, chronic pain, and major depressive disorder presents to the emergency department with altered mental status. His wife found him minimally responsive on the couch. She does not know which medications he normally takes but he has been particularly depressed lately after the death of his brother. His temperature is 97.8°F (36.6°C), blood pressure is 89/58 mmHg, pulse is 102/min, respirations are 16/min, and oxygen saturation is 100% on room air. Physical exam is notable for a patient who flexes his extremities in response to sternal rub. The patient's pupils are 3 mm bilaterally and reactive to light. Laboratory values are ordered as seen below.

Serum:
Na+: 140 mEq/L
Cl-: 102 mEq/L
K+: 4.1 mEq/L
HCO3-: 24 mEq/L
BUN: 20 mg/dL
Glucose: 112 mg/dL
Creatinine: 1.1 mg/dL
Ca2+: 10.2 mg/dL
Alcohol: 300 mg/dL

The patient is intubated and admitted to the intensive care unit. Roughly 24 hours later in the intensive care unit, the patient is still unable to be extubated given his lack of respiratory drive. Which of the following is the most likely etiology of this patient's symptoms?