Nutritional skin disease: practice question

A Dermatology 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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Dermatology · Nutritional skin disease

Sample item

A patient with tuberculosis on isoniazid without pyridoxine supplementation develops a photosensitive, scaly, pigmented rash over the neck and dorsum of the hands, along with diarrhoea and confusion. What is the mechanism?

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

The answer and the reasoning

B. Impaired conversion of tryptophan to niacin due to pyridoxine antagonism, causing pellagra

Isoniazid antagonises pyridoxine, which is a cofactor in the conversion of tryptophan to niacin, so deficiency leads to pellagra with dermatitis (Casal's necklace on the neck), diarrhoea and dementia. Pyridoxine supplementation prevents this and the associated peripheral neuropathy, and treatment of pellagra is nicotinamide.

Option by option

  • A. Vitamin C deficiency due to ethambutol

    Scurvy causes perifollicular haemorrhages and bleeding gums.

  • B. Impaired conversion of tryptophan to niacin due to pyridoxine antagonism, causing pellagra (correct)

    Correct. A secondary niacin deficiency from reduced pyridoxine-dependent synthesis causes pellagra.

  • C. Direct photosensitisation by rifampicin in the skin

    Rifampicin colours body fluids orange and does not cause pellagra.

  • D. Zinc deficiency from chelation by pyrazinamide

    Zinc deficiency causes acrodermatitis enteropathica with perioral and acral lesions.

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 Dermatology question

What is the answer to this Dermatology question?

The best answer is B, Impaired conversion of tryptophan to niacin due to pyridoxine antagonism, causing pellagra. Isoniazid antagonises pyridoxine, which is a cofactor in the conversion of tryptophan to niacin, so deficiency leads to pellagra with dermatitis (Casal's necklace on the neck), diarrhoea and dementia.

Why is "Vitamin C deficiency due to ethambutol" not the answer?

Scurvy causes perifollicular haemorrhages and bleeding gums.

Why is "Direct photosensitisation by rifampicin in the skin" not the answer?

Rifampicin colours body fluids orange and does not cause pellagra.

Why is "Zinc deficiency from chelation by pyrazinamide" not the answer?

Zinc deficiency causes acrodermatitis enteropathica with perioral and acral lesions.

What concept does this question test?

It tests Nutritional skin disease in Dermatology. Pyridoxine supplementation prevents this and the associated peripheral neuropathy, and treatment of pellagra is nicotinamide.

How hard is this question?

We tag it as a multi-step question: it asks you to combine more than one idea. 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 Nutritional skin disease?

Learn each condition by its primary lesion, site and signature sign. Then use the flashcards and the other Dermatology 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.