AKT-5282

A 50-year-old woman presents to her General Practitioner with a 2-month history of progressive ataxia and dysarthria. She had a right mastectomy three years ago for breast cancer. On examination, there is gaze-evoked nystagmus, dysarthria, upper limb ataxia and gait ataxia. Power, tone, reflexes and sensation are normal. Plantar response is flexor bilaterally. Computed tomography (CT), magnetic resonance imaging brain (MRI) and cerebrospinal fluid (CSF) analysis are normal.
What is the single most likely cause of this patient’s ataxia?