Carcinoid syndrome occurs in neuroendocrine tumors when serotonin and vasoactive peptides reach the systemic circulation. In which clinical scenario does carcinoid syndrome most commonly occur?

• A Any primary small bowel NET regardless of metastatic status
• B Primary bronchial carcinoid without hepatic metastases
• C Midgut NETs with liver metastases, bypassing first-pass hepatic metabolism ✓
• D Foregut NETs with extensive mesenteric nodal metastases

Explanation

Carcinoid syndrome requires that vasoactive amines (principally serotonin) and peptides reach the systemic circulation without being metabolized. Hepatic portal circulation metabolizes serotonin efficiently; therefore, carcinoid syndrome occurs when the primary tumor drains directly into systemic circulation (bronchial, ovarian carcinoids) OR when liver metastases from midgut NETs allow direct release into hepatic veins. The classic presentation (flushing, diarrhea, bronchospasm, cardiac fibrosis — Hedinger syndrome) is seen in hepatic-metastatic midgut NETs. Primary small bowel NETs without metastases rarely cause carcinoid syndrome.

Reference: Bailey & Love's Short Practice of Surgery, 27th ed.

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