Vitamins: niacin and pellagra: 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 · Vitamins: niacin and pellagra

Sample item

A farmer from a village whose staple is maize develops a symmetrical photosensitive rash over the neck (Casal necklace), diarrhoea and recent confusion. What best explains his condition?

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

The answer and the reasoning

D. Bound niacin (niacytin) and low tryptophan in maize limit NAD synthesis

Pellagra (dermatitis, diarrhoea, dementia) results from niacin deficiency. In maize, niacin is bound and not bioavailable unless treated with alkali, and the protein is low in tryptophan, the precursor from which the body makes about 1 mg niacin from 60 mg tryptophan. Treatment is nicotinamide.

Option by option

  • A. Maize contains avidin, which sequesters niacin

    Avidin is found in raw egg white and binds biotin, not niacin.

  • B. Maize lacks pyridoxine only, which blocks conversion of tryptophan to NAD

    Maize is not selectively lacking pyridoxine, and B6 deficiency causes neuropathy and sideroblastic anaemia.

  • C. Maize is deficient in thiamine, causing pyruvate accumulation

    Thiamine deficiency causes beriberi and Wernicke encephalopathy, not this triad.

  • D. Bound niacin (niacytin) and low tryptophan in maize limit NAD synthesis (correct)

    Unavailable bound niacin plus low tryptophan leaves too little NAD and NADP for tissues with high turnover.

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 D, Bound niacin (niacytin) and low tryptophan in maize limit NAD synthesis. Pellagra (dermatitis, diarrhoea, dementia) results from niacin deficiency.

Why is "Maize contains avidin, which sequesters niacin" not the answer?

Avidin is found in raw egg white and binds biotin, not niacin.

Why is "Maize lacks pyridoxine only, which blocks conversion of tryptophan to…" not the answer?

Maize is not selectively lacking pyridoxine, and B6 deficiency causes neuropathy and sideroblastic anaemia.

Why is "Maize is deficient in thiamine, causing pyruvate accumulation" not the answer?

Thiamine deficiency causes beriberi and Wernicke encephalopathy, not this triad.

What concept does this question test?

It tests Vitamins: niacin and pellagra in Biochemistry. In maize, niacin is bound and not bioavailable unless treated with alkali, and the protein is low in tryptophan, the precursor from which the body makes about 1 mg niacin from 60 mg tryptophan. Treatment is nicotinamide.

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 Vitamins: niacin and pellagra?

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.