This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Motor System, Subject name: Physiology
A 60-year-old man has resting tremor, pill-rolling movements, rigidity, and bradykinesia. Which of the following is most likely to be decreased in this man? 
Select and explain. 
A. Serotonin neurons in the raphe nuclei, B. GABA neurons in the caudate nucleus and putamen, C. Dopamine neurons in the substantia nigra, D. Acetylcholine neurons in the forebrain
C: Dopamine neurons in the substantia nigra
Answer: C: Dopamine neurons in the substantia nigra 
Explanation: The clinical features points toward a diagnosis of Parkinson disease. It is characterized by loss of dopaminergic neurons that project from the substantia nigra to the putamen. Other options: Serotonergic raphe neurons have been implicated in the control of mood and descending pain modulation (analgesia) pathway. GABAergic pathway loss from striatum (caudate nucleus + putamen) results in hyperkinetic disorder, Huntington disease. Loss of nucleus basalis of Meyne in the basal forebrain, a diffuse cholinergic projection to the entire neocoex, the amygdala, and the hippocampus leads to Alzheimer disease.