Pellagra is due to the deficiency of . . . . . . . .

Pellagra is due to the deficiency of Niacin.
Pellagra is a disease caused by a lack of the vitamin niacin (vitamin B₃). Symptoms include inflamed skin, diarrhea, dementia and sores in the mouth.
The classic symptoms of pellagra are diarrhea, dermatitis, dementia and death.
Treatment is with either niacin or nicotinamide supplementation. Improvements typically begin within a couple of days.