Snapshot A 12-year-old boy is brought to his pediatrician by his mother due to progressive confusion, diarrhea, and an itchy rash. He recovered from the flu about a week ago but was still feeling tired. Over the last 48 hours, he has had several episodes of diarrhea per day. On exam, he has a tremor, intermittent ataxia, and a rash (shown in the photograph). His mom says that he has previously had these kinds of symptoms usually after he has an illness. He is given nicotinic acid and placed on a high protein diet and quickly recovers. Introduction Amino acid clearance amino acids are freely filtered by the glomerulus into Bowman's capsule filtered amino acids are reabsorbed in the proximal tubule by sodium dependent co-transporters that are separated into login to view 3 more bullets in normal physiology all amino acids are reabsorbed by the tubule meaning that clearance of amino acids is zero amino acids are preserved for further use by the body defective reabsorption of amino acids by the proximal tubule can lead to Hartnup disease Hartnup deficiency deficiency of proximal renal tubule neutral amino acid transporters pathogenesis defective reabsorption of neutral amino acids leading to login to view 2 more bullets genetics login to view 1 more bullet prognosis login to view 2 more bullets Presentation Symptoms pellagra with onset in childhood consisting of diarrhea dementia dermatitis failure to thrive episodes triggered by infections or other stressors Physical exam red rash usually due to photosensitivity ataxia during episodes of pellagra Studies Serologies ↓ tryptophan Labs ↑ neutral amino acids in urine Differential Pellagra due to vitamin deficiency Malignant carcinoid syndrome Treatment Conservative high protein diet indication login to view 2 more bullets nicotinic acid indication login to view 1 more bullet rationale login to view 3 more bullets Complications Complications neurologic degeneration if pellagra is not treated properly