Carcinoid syndrome is due to: March 2013
**Core Concept**
Carcinoid syndrome is a clinical disorder caused by the excessive release of vasoactive substances, primarily serotonin (5-HT), from a carcinoid tumor. This condition is often associated with metastatic carcinoid tumors, particularly those originating in the gastrointestinal tract.
**Why the Correct Answer is Right**
The carcinoid tumor secretes serotonin through a process called enterochromaffin secretion, which is mediated by the enzyme tryptophan hydroxylase. The excess serotonin is then released into the systemic circulation, leading to a constellation of symptoms including flushing, diarrhea, wheezing, and heart valve abnormalities. The symptoms are caused by the vasoactive properties of serotonin, which induce vasodilation and bronchospasm.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because carcinoid syndrome is not primarily caused by the release of catecholamines, which are responsible for other types of endocrine disorders.
**Option B:** This option is incorrect because histamine is not the primary substance responsible for the symptoms of carcinoid syndrome.
**Option C:** This option is incorrect because gastrin is a hormone associated with peptic ulcer disease, not carcinoid syndrome.
**Clinical Pearl / High-Yield Fact**
Carcinoid syndrome can be diagnosed by measuring the urinary excretion of 5-HIAA (5-hydroxyindoleacetic acid), the primary metabolite of serotonin. This test is a useful screening tool for patients with suspected carcinoid syndrome.
**Correct Answer: D. The excessive release of serotonin from a carcinoid tumor.**