Final answer:
Pellagra in carcinoid syndrome is caused by niacin deficiency due to the overproduction of serotonin by carcinoid tumors, which use up tryptophan that is otherwise needed for niacin synthesis. This differs from pernicious anemia, which involves a deficit in vitamin B12 absorption.
Step-by-step explanation:
The cause of pellagra in carcinoid syndrome relates to the overproduction of serotonin. Carcinoid tumors, which are a type of neuroendocrine tumor, often secrete a variety of substances, including serotonin. This excessive secretion of serotonin can lead to a deficiency in the vitamin niacin (vitamin B3), because tryptophan, which is normally used by the body to make niacin, is instead being used to produce the large amounts of serotonin. Without adequate niacin, the body cannot maintain healthy skin, the digestive system, and nervous system functions, leading to the development of pellagra. This is distinct from pernicious anemia, which is related to vitamin B12 deficiency and poor absorption, often seen in patients with conditions that affect the intestines or stomach, such as Crohn's disease, surgical removal of parts of the intestine or stomach, intestinal parasites, and AIDS.