A 67-year-old male with a history of hypertension and atrial fibrillation is brought to the emergency department by his daughter, who reports that over the past 48 hours he has become increasingly confused, emotionally labile with episodes of crying without clear provocation, and has been making bizarre statements. She initially thought he was having a psychiatric crisis. On examination, the patient has a flattened right nasolabial fold, subtle right-sided pronator drift, and his speech is fluent but contains occasional paraphasic errors. He is alert but disoriented to date and situation. His vital signs show blood pressure 178/96 mmHg and an irregularly irregular heart rhythm at 92 bpm. A depression screening is attempted but he is unable to reliably complete it due to confusion. Which of the following diagnoses should be prioritized in the immediate workup?