Vertigo

A 30-year-old woman comes to the clinic due to spinning sensation and feeling unstable. This sensation started 1 month ago and since then has gotten gradually worse. She also describes constant fatigue and tingling sensation in both of her upper extremities. She states that several months ago, she went to the emergency department for an episode of intense right eye pain with movement and the loss of color vision; this gradually improved over 2 weeks. Vital signs are within normal limits. Physical examination shows ataxia and nystagmus. A head MRI shows multiple hyperintense periventricular and cerebellar ovoid lesions. Which of the following is the most likely additional finding in this patient?