## **Core Concept**
Carcinoid tumors are a type of neuroendocrine tumor that can arise from various sites, including the gastrointestinal tract and lungs. They are known for their ability to secrete various bioactive substances, leading to the carcinoid syndrome in some cases. The primary sites of carcinoid tumors include the appendix, ileum, and lungs.

## **Why the Correct Answer is Right**
The statement about carcinoid tumors that is not true needs to be identified based on the characteristics of these tumors. Carcinoid tumors indeed commonly occur in the appendix and ileum, and they can metastasize to the liver. They are also known to occur in the lungs. The key is understanding the frequency of metastasis and the symptoms they cause.

## **Why Each Wrong Option is Incorrect**
- **Option A:** 80% occur in appendix and ileum - This statement is generally true, as a significant proportion of carcinoid tumors do occur in these locations.
- **Option B:** Most of them metastasize to liver - This is also true, particularly for tumors originating in the ileum and other parts of the gastrointestinal tract beyond the appendix.
- **Option D:** They can occur in the lungs - This is correct; carcinoid tumors are known to occur in the lungs, representing a significant proportion of lung neuroendocrine tumors.

## **Why Option C is Incorrect**
- **Option C:** 90% of them cause carcinoma symptoms - This statement is not true. Only a small percentage of carcinoid tumors cause the carcinoid syndrome, which is characterized by symptoms such as flushing, diarrhea, wheezing, and heart valve abnormalities. This syndrome typically occurs when liver metastases are present, allowing the secreted substances to bypass liver metabolism and directly enter the systemic circulation.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that the carcinoid syndrome is much more likely to occur with liver metastasis or when the primary tumor is located outside the gastrointestinal tract (e.g., in the lungs), as the liver normally metabolizes the substances secreted by these tumors.

## **Correct Answer: C. 90 % of them cause carcinoma symptoms**