Soru

A 29-year-old man is brought to the emergency department by his partner due to the sudden onset of right-sided weakness and difficulty speaking that began $2$ hours ago. Over the past $6$ months, the patient has experienced profound fatigue and intermittent episodes of difficulty swallowing. He also reports vague, episodic abdominal pain and passing very dark urine, which he notes is most prominent during his first void of the morning. His temperature is $37.0^\circ\text{C}$ ($98.6^\circ\text{F}$), blood pressure is $132/82 \text{ mm Hg}$, pulse is $98/\text{min}$, and respiratory rate is $16/\text{min}$. Physical examination reveals mild scleral icterus, right-sided hemiparesis, and expressive aphasia. The abdomen is soft and non-tender. Laboratory studies show:

Hemoglobin: $7.8 \text{ g/dL}$
Mean corpuscular volume: $99 \text{ fL}$
Platelet count: $122,000/\text{mm}^3$
Leukocyte count: $3,800/\text{mm}^3$
Lactate dehydrogenase: $2,100 \text{ U/L}$
Total bilirubin: $3.8 \text{ mg/dL}$
Direct bilirubin: $0.5 \text{ mg/dL}$
Haptoglobin: $<10 \text{ mg/dL}$
Creatinine: $1.3 \text{ mg/dL}$
Direct antiglobulin (Coombs) test: Negative

A non-contrast CT scan of the head shows no acute intracranial hemorrhage. Which of the following is the most appropriate diagnostic test to establish the underlying etiology of this patient's presentation?