MRCP2-0801

MRCP2-0801

A 28-year-old male from Eastern Europe presents to the Emergency Department with 72 hours of severe ‘burning and sharp’ electrical pains in his arms and legs. He is well known to the department, having been treated for alcohol withdrawal a number of times in the past and is a known intravenous drug user. He reports no weakness, dysarthria or hallucinations. He reports drinking about 8 pints of beer in the past 24 hours, which he says is normal for him, and has used intravenous heroin daily for the past 3 weeks.

On examination, both upper and lower limbs are rigid and mildly bradykinetic. A bilateral resting tremor is noted in both hands. Reflexes and sensation are normal. Examination of cranial nerves and eye movements are unremarkable.

His initial blood tests return and are as follows:

Hb 162 g/l
Platelets 280 * 109/l
WBC 5.8 * 109/l
Vitamin B12 198 ng/l
Folate 480 (>317 nmol/l)

Na+ 141 mmol/l
K+ 4.2 mmol/l
Urea 6.2 mmol/l
Creatinine 87 µmol/l
CRP 8 mg/l
HIV negative

Which investigation is most likely to yield the underlying diagnosis?