Amino acid transport disorders: Hartnup disease: 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 transport disorders: Hartnup disease

Sample item

A 9-year-old boy has episodes of a photosensitive pellagra-like rash and intermittent cerebellar ataxia. Urine shows excess neutral amino acids such as alanine and glutamine, but not cystine or lysine. Which defect is most likely?

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

The answer and the reasoning

C. A defective neutral amino acid transporter in intestine and renal tubule

Hartnup disease is an autosomal recessive defect of the sodium-dependent neutral amino acid transporter SLC6A19 in the brush border of the small intestine and proximal renal tubule. Poor absorption of tryptophan reduces the supply for niacin synthesis, so a pellagra-like illness appears, particularly during poor nutrition. A high-protein diet and nicotinamide are effective.

Option by option

  • A. A defective dibasic amino acid transporter in the renal tubule

    A dibasic transporter defect gives excess cystine, ornithine, lysine and arginine, as in cystinuria.

  • B. Deficiency of tryptophan hydroxylase

    Tryptophan hydroxylase deficiency would reduce serotonin and does not produce a neutral aminoaciduria.

  • C. A defective neutral amino acid transporter in intestine and renal tubule (correct)

    Correct. Loss of neutral amino acid uptake causes neutral aminoaciduria and tryptophan-dependent niacin deficiency.

  • D. Deficiency of kynureninase in the liver

    Kynureninase deficiency would not cause amino acid loss in urine of a neutral type.

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 C, A defective neutral amino acid transporter in intestine and renal tubule. Hartnup disease is an autosomal recessive defect of the sodium-dependent neutral amino acid transporter SLC6A19 in the brush border of the small intestine and proximal renal tubule.

Why is "A defective dibasic amino acid transporter in the renal tubule" not the answer?

A dibasic transporter defect gives excess cystine, ornithine, lysine and arginine, as in cystinuria.

Why is "Deficiency of tryptophan hydroxylase" not the answer?

Tryptophan hydroxylase deficiency would reduce serotonin and does not produce a neutral aminoaciduria.

Why is "Deficiency of kynureninase in the liver" not the answer?

Kynureninase deficiency would not cause amino acid loss in urine of a neutral type.

What concept does this question test?

It tests Amino acid transport disorders: Hartnup disease in Biochemistry. Poor absorption of tryptophan reduces the supply for niacin synthesis, so a pellagra-like illness appears, particularly during poor nutrition. A high-protein diet and nicotinamide are effective.

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 transport disorders: Hartnup disease?

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.