Gastrointestinal: carcinoid syndrome: practice question

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Original practice item, not a recalled exam question. Nothing here is recorded.

The answer and the reasoning

A. Serotonin from the gut primary is degraded by hepatic monoamine oxidase on first pass, while metastases release it directly into the hepatic veins

Serotonin and other vasoactive products from midgut carcinoids reach the liver through the portal vein and are inactivated by monoamine oxidase. Once metastases are present, the products drain through hepatic veins into the systemic circulation. Bronchial or ovarian carcinoids can cause the syndrome without liver metastases because they bypass the portal circulation.

Option by option

• A. Serotonin from the gut primary is degraded by hepatic monoamine oxidase on first pass, while metastases release it directly into the hepatic veins (correct)

  Correct. First-pass hepatic inactivation is bypassed once metastases drain into the hepatic veins.

• B. Liver metastases compress the portal vein and divert gut blood into the systemic circulation

  Portal compression is not the mechanism, and the flushing is caused by circulating mediators.

• C. Hepatic enzymes convert the inactive peptide from the primary tumour into active serotonin

  Serotonin is not made by hepatic activation of an inactive peptide, as it is synthesised from tryptophan in the tumour cells.

• D. Hepatic metastases produce serotonin that the primary tumour cannot synthesise

  Primary carcinoid cells do synthesise serotonin, so this is not about a new synthetic capacity.

Source note: Original NextPrep sample item — pending expert review. This item is original practice material awaiting independent expert review, so confirm important facts in a standard textbook.

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