A patient with carcinoid syndrome develops pellagra despite adequate dietary niacin intake. Which metabolic mechanism explains this finding?

• A Carcinoid tumors consume circulating niacin for rapid cell proliferation
• B Excessive serotonin production diverts tryptophan away from niacin synthesis ✓
• C Carcinoid syndrome causes malabsorption of niacin in the jejunum
• D Carcinoid tumors overexpress NAD glycohydrolase, degrading NAD

Explanation

In carcinoid syndrome, the tumor shunts large amounts of tryptophan toward serotonin synthesis via tryptophan hydroxylase, depleting the substrate pool available for niacin biosynthesis via the kynurenine pathway. This causes pellagra despite adequate dietary niacin. Hartnup disease causes pellagra through impaired tryptophan absorption. Malabsorption and NAD degradation are not recognized mechanisms.

Reference: Robbins and Cotran Pathologic Basis of Disease, 10th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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