MRCP2-3031

MRCP2-3031

A 50-year-old woman presents to the Medical Admission Unit with a 12-hour history of weakness. Two days prior to admission, she experienced double vision and increased unsteadiness while walking. On examination, a mass is found in her abdomen and she has reduced muscle tone and power, absent reflexes, and an ataxia. Initial investigations reveal normal blood counts, electrolytes, and liver function tests, but an elevated CRP. A CT brain scan is normal. What is the most useful investigation to reach a diagnosis?