# Hartnup Disease

**Summary**

Hartnup Disease is an autosomal recessive metabolic disorder characterized by a dysfunction in the neutral amino acid transporter. This defect causes decreased absorption of nonpolar amino acids, specifically tryptophan. Since tryptophan is important for the synthesis of [Vitamin B3 (Niacin)](/content/view/3716/index.html), patients with Hartnup disease will also present with signs and symptoms consistent with [Niacin Deficiency](/content/view/3816/index.html), with pellagra-like symptoms. Since clinical findings are primarily due to niacin/tryptophan deficiency, niacin supplementation and/or a high-protein diet can be used to treat the disease.

**Key Points**

- **Hartnup Disease**
  - Autosomal recessive inheritance
  - Deficiency of **neutral amino acid** (eg, **tryptophan**) **transporters** in proximal renal tubular cells and on enterocytes
    - Increased excretion of tryptophan in urine and decreased absorption from the gut → **decreased tryptophan levels** → decreased conversion to niacin → **pellagra-like** symptoms
  - Treat with high-protein diet and nicotinic acid (niacin)

Find Hartnup Disease and other [Vitamins](/content/browse#playlist-2700/index.html) among Pixorize's visual mnemonics for the USMLE Step 1 and NBME shelf exams.