What is the main disorder associated with niacin deficiency?

The main disorder associated with niacin deficiency is pellagra. Pellagra is characterized by the "three D's": dermatitis, diarrhea, and dementia. These symptoms arise due to the essential role of niacin in energy metabolism and in the synthesis of important molecules, including those involved in DNA repair and the production of neurotransmitters.

Niacin, also known as vitamin B3, is crucial for converting food into energy and maintaining healthy skin, nerves, and digestion. When there is a deficiency of niacin, these processes are disrupted, leading to the symptoms associated with pellagra. Historical outbreaks of pellagra have typically occurred in populations with poor dietary intake of niacin and tryptophan, which the body can convert to niacin.

Other options listed pertain to different nutrient deficiencies: anemia is often linked to deficiencies in iron, vitamin B12, or folate; scurvy is associated with a deficiency in vitamin C; and beriberi relates to thiamine (vitamin B1) deficiency. The clear association between niacin specifically and pellagra makes it the correct answer.