Russell is 72 years old and is still working part-time as a professor. He has noticed some arm and hand shaking in the past year, but assumed it must be related to low or high blood sugar levels because he has had some issues with that in the past. However, the shaking/twitching has become more consistent lately and does not seem to be correlated with his diet. Russell's doctor said that the stress of his job could be the problem, so he took the entire summer off from teaching.
When he returned to work in the fall, Russell and his students noticed that his handwriting has become nearly illegible. Drinking a cup of coffee without spilling had also become a challenge. Russell returned to his doctor, who performed a physical exam and a few tests. Here are the notable results: shuffling gait, mild bradykinesia, mild voice tremor, intermittent rigidity of the limbs, and normal EEG.
If Russell is having symptoms in his upper extremities, lower extremities, and his speech is slurred, which of the following body regions is most likely experiencing a pathology?

a. Brain and spinal cord
b. Cranial nerves
c. Peripheral nerves
d. Spinal nerves