A 12-year-old develops scaly erythematous rash on sun-exposed skin, intermittent cerebellar ataxia, and diarrhea after eating a protein-rich meal. Neutral amino acids including tryptophan are massively excreted in urine while plasma levels remain normal. What is the underlying defect?
- A Defective renal reabsorption of cystine and dibasic amino acids only
- B Defective intestinal and renal reabsorption of neutral amino acids ✓
- C Defect in tryptophan conversion to niacin due to vitamin B6 deficiency
- D Defective intestinal absorption of monosaccharides
Correct answer: B. Defective intestinal and renal reabsorption of neutral amino acids
Explanation
Hartnup disease is caused by a defect in the neutral amino acid transporter of intestinal brush border and renal tubules. Urinary loss of tryptophan depletes niacin synthesis, producing a pellagra-like rash, ataxia, and diarrhea that respond to nicotinamide supplementation. Option A describes cystinuria, which involves dibasic amino acids and causes stones, not pellagra.
Reference: Harper's Illustrated Biochemistry, 32nd ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.