An adolescent develops episodic cerebellar ataxia after sun exposure, along with pellagra-like dermatitis and diarrhea that respond poorly to nicotinamide alone. Neutral aminoaciduria affects alanine, serine, valine, phenylalanine, and tryptophan. The underlying defect involves:

• A Sodium-dependent neutral amino acid transport by the SLC6A19 (B0AT1) transporter in kidney and intestine ✓
• B Renal and intestinal absorption of cystine and dibasic amino acids via the b0,+ transporter
• C Tryptophan conversion to niacin via the kynurenine pathway
• D Intestinal peptide hydrolysis due to enterokinase deficiency

Explanation

Hartnup disease is caused by mutations in SLC6B19 encoding the intestinal and renal brush border neutral amino acid transporter A0AT1, producing neutral aminoaciduria with loss of tryptophan. Secondary niacin deficiency causes the pellagra-like rash and neurologic symptoms, hence nicotinamide supplementation helps. Option B describes cystinuria, which causes stones but no dermatitis or ataxia. Option C is the pathway whose reduced flux causes the symptoms, not the primary defect. Enterokinase deficiency causes protein malabsorption, not specific aminoaciduria.

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.