A 9-year-old boy has episodes of cerebellar ataxia and a pellagra-like photosensitive rash on sun-exposed skin. Urine shows generalized aminoaciduria including neutral amino acids, but cystine excretion is normal and there is no renal stone history. Oral nicotinamide improves both rash and neurologic symptoms. Defect in which protein explains this disorder?

• A Cystine-dibasic amino acid antiporter rBAT
• B SLC6A19 neutral amino acid transporter in proximal tubule and intestine ✓
• C Intestinal peptide transporter PEPT1
• D Renal sodium-phosphate cotransporter

Explanation

Hartnup disease arises from defective SLC6B19 (A0AT1), the brush-border neutral amino acid transporter of kidney and intestine. Massive urinary loss of tryptophan depletes niacin reserves, producing pellagra-like dermatitis, ataxia, and neuropsychiatric symptoms that respond to nicotinamide supplementation. Cystinuria involves the dibasic amino acid transporter rBAT and causes recurrent stones, which distinguishes it here by the absence of urolithiasis and the broad neutral aminoaciduria.

Reference: Harper's Illustrated Biochemistry, 32nd ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.