Cerebrovascular Disease USMLE Step 1 Practice Question

A 58-year-old woman with a 2-year history of untreated atrial fibrillation presents to the emergency department with acute onset of slurred speech and right-sided weakness. She reports a 3-day history of progressive difficulty with concentration and a 24-hour history of involuntary limb jerking. She denies fever, headache, or recent illness. Vital signs: BP 142/86 mmHg, HR 98 bpm (irregular), RR 18, temperature 37.1°C. Neurologic examination reveals dysarthria, right hemiparesis (4/5), and bilateral asterixis. Noncontrast head CT is unremarkable. MRI brain with diffusion-weighted imaging shows multiple acute infarcts scattered across anterior and posterior circulation territories. Laboratory studies reveal INR 1.2, negative blood cultures, normal CSF glucose and protein (pending cultures), and normal CBC. Which of the following is the most important next step in diagnosis?