Dementia, dermatitis, diarrhoea are features of

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.