Final answer:
Serotonin is the neurotransmitter most likely involved in the mood disturbances and appetite problems experienced by Andy.
Step-by-step explanation:
Based on the symptoms of mood disturbances and appetite problems, the neurotransmitter most likely involved is serotonin. Serotonin is known to regulate mood, sleep, and appetite. Low levels of serotonin in the brain are associated with depression, and medications called antidepressants help bring serotonin levels back to normal.