Amino acid metabolism: tryptophan in carcinoid syndrome: practice question

A Biochemistry question in the style of the NEET PG paper. Answer it first, then read why the right option is right and why each of the others is wrong.

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Biochemistry · Amino acid metabolism: tryptophan in carcinoid syndrome

Sample item

A 55-year-old man with metastatic midgut carcinoid has flushing, diarrhoea and wheeze. His 24-hour urinary 5-HIAA is raised. Over months he also develops a scaly pigmented dermatitis on sun-exposed skin, glossitis and mild confusion. Which mechanism best explains the dermatitis?

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

The answer and the reasoning

B. Tryptophan diverted to serotonin, reducing niacin synthesis

About 1 percent of dietary tryptophan normally becomes serotonin, and part of the rest is converted through the kynurenine pathway to niacin. In carcinoid syndrome large tumour mass diverts tryptophan into serotonin and 5-HIAA, causing pellagra with dermatitis, diarrhoea and dementia. Nicotinamide supplementation can correct it.

Option by option

  • A. Defective intestinal absorption of neutral amino acids from a transporter mutation

    A transporter defect of Hartnup type is inherited and presents in childhood, not with a carcinoid tumour.

  • B. Tryptophan diverted to serotonin, reducing niacin synthesis (correct)

    Correct. Tryptophan diversion leaves too little for NAD synthesis, causing pellagra.

  • C. Excess tryptophan converted to melatonin

    Melatonin is derived from serotonin and its production does not explain tryptophan shortage.

  • D. Impaired conversion of phenylalanine to tyrosine

    Phenylalanine hydroxylase impairment does not explain pellagra-type dermatitis.

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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Questions about this Biochemistry question

What is the answer to this Biochemistry question?

The best answer is B, Tryptophan diverted to serotonin, reducing niacin synthesis. About 1 percent of dietary tryptophan normally becomes serotonin, and part of the rest is converted through the kynurenine pathway to niacin.

Why is "Defective intestinal absorption of neutral amino acids from a transpo…" not the answer?

A transporter defect of Hartnup type is inherited and presents in childhood, not with a carcinoid tumour.

Why is "Excess tryptophan converted to melatonin" not the answer?

Melatonin is derived from serotonin and its production does not explain tryptophan shortage.

Why is "Impaired conversion of phenylalanine to tyrosine" not the answer?

Phenylalanine hydroxylase impairment does not explain pellagra-type dermatitis.

What concept does this question test?

It tests Amino acid metabolism: tryptophan in carcinoid syndrome in Biochemistry. In carcinoid syndrome large tumour mass diverts tryptophan into serotonin and 5-HIAA, causing pellagra with dermatitis, diarrhoea and dementia. Nicotinamide supplementation can correct it.

How hard is this question?

We tag it as an application question: it asks you to use a fact on a patient or a finding. The tag is our own editorial judgement, not an official difficulty rating, so read the reason for every option, not only the answer.

How should I study Amino acid metabolism: tryptophan in carcinoid syndrome?

Build one table of enzyme deficiency, disease, inheritance and key finding, then revise it by spaced recall. Then use the flashcards and the other Biochemistry questions on this site to test what stuck.

Is this a real NEET PG question?

No. It is an original practice item (editorial status: sample, version 1). NextPrep does not publish recalled or leaked questions, and independent faculty review of this item has not been completed yet, so confirm important facts in a standard textbook.