Final answer:
The neurotransmitter most likely to be underperforming in Joseph's brain, who has been diagnosed with Alzheimer's disease, is acetylcholine.
Step-by-step explanation:
Jospeh, who has been diagnosed with Alzheimer's disease, is most likely experiencing an underperformance of the neurotransmitter acetylcholine. In the brains of individuals with Alzheimer's, there is a decrease in the activity of cholinergic neurons, which utilize acetylcholine to communicate. Many treatments for this disease, including drugs like Aricept, work by enhancing acetylcholine neurotransmission; they inhibit an enzyme called acetylcholinesterase, which normally breaks down acetylcholine, thereby increasing its availability and prolonging neural function.