A 76 year old man attends clinic with reduced mobility for the last 3 months. As he walks into the clinic room, you see he has a shuffling gait with reduced arm swing. On examination, there is rigidity, more so in the left upper limb than the right. There is no tremor but you think there is bradykinesia. He is on a long list of medication and you are not sure if they might be causing some of the symptoms. Which of the following investigations could you use to support the likely diagnosis?