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What is Dementia, dermatitis, diarrhoea are features of and explain in detail?
Deficiency of niacin causes pellagra, a disease involving the skin, gastrointestinal tract, and CNS. 
The symptoms of pellagra progress through the three Ds: dermatitis (photosensitive), diarrhea, and dementia. If untreated, death (a fourth D) occurs. 
Hartnup disorder, characterized by defective absorption of tryptophan, can result in pellagra-like symptoms.

Ref : Lippincott’s illustrated reviews for biochemistry , 7th edition.