Carcinoid tumor secretes
**Core Concept**
Carcinoid tumors are a type of neuroendocrine tumor that can secrete various bioactive substances, leading to a range of systemic symptoms. These tumors arise from the enterochromaffin cells of the gastrointestinal tract, which are part of the diffuse neuroendocrine system. The secreted substances can cause significant morbidity and mortality.
**Why the Correct Answer is Right**
The most common substance secreted by carcinoid tumors is serotonin (5-hydroxytryptamine, 5-HT), which is synthesized by the tumor cells from the amino acid tryptophan. Serotonin is stored in dense-core vesicles and released into the bloodstream, where it can cause a range of systemic symptoms, including flushing, diarrhea, wheezing, and heart valve abnormalities. The release of serotonin is often regulated by the tumor's ability to express and activate the enzyme tryptophan hydroxylase, which is essential for serotonin synthesis.
**Why Each Wrong Option is Incorrect**
**Option A:** Histamine is not a primary substance secreted by carcinoid tumors, although some may express histidine decarboxylase, the enzyme responsible for its synthesis.
**Option B:** Adrenaline is not a typical product of carcinoid tumors, which derive from the enterochromaffin cells of the gastrointestinal tract rather than the adrenal medulla.
**Option C:** Gastrin is a peptide hormone secreted by G cells in the stomach, not carcinoid tumors.
**Clinical Pearl / High-Yield Fact**
Carcinoid syndrome, characterized by flushing, diarrhea, wheezing, and heart valve abnormalities, is a classic presentation of carcinoid tumors, particularly those that metastasize to the liver.
**Correct Answer:** D. Serotonin