Final answer:
Pellagra is caused by a deficiency of niacin (vitamin B3). It is characterized by dermatitis, dementia, diarrhea, and can be fatal if untreated.
Step-by-step explanation:
The deficiency that causes pellagra, which is characterized by dermatitis, dementia, diarrhea, and death, is a deficiency of niacin (also known as vitamin B3). When a person lacks niacin, they may experience symptoms such as inflammation and irritation of the skin (dermatitis), mental confusion and memory loss (dementia), and gastrointestinal issues such as diarrhea. If left untreated, pellagra can be fatal.