MRCP2-3089

A 40-year-old man visits his primary care physician with a history of occasional difficulty speaking and reduced facial expressions. He also reports occasional regurgitation of food through his nose, which he finds embarrassing. His symptoms are most severe at the end of the day. He has no significant medical history and takes no regular medications.
During the examination, his vital signs are normal. His heart and lungs are functioning normally, and he has no motor or sensory deficits. He reports blurred vision during visual field testing.
What is the initial investigation to request, given the probable underlying diagnosis?