A patient with "the 4 D's" of dermatitis, diarrhea, and dementia and eventually death; is experiencing symptoms of lacking niacin (B3) and is suffering with is called Primary Pellagra. They are not getting enough niacin and tryptophan in their daily diet.
Secondary Pellagra is usually due to not being able to withstand niacin in a diet. Generally, alcoholism; severe diarrhea to the point of severe dehydration; and certain medications will cause this to happen.