Definition of Hartnup disorder:

Hartnup disorder is an inherited metabolic disorder involving the transport of certain amino acids (for example, tryptophan and histidine) in the small intestine and kidneys.

Causes, incidence, and risk factors:

Hartnup disorder is possibly the most common metabolic condition involving amino acids. It is a genetic disorder inherited in an autosomal recessive manner, which means that the child must inherit the defective gene from both parents in order to be seriously affected.

Although most people show no symptoms, photosensitivity (sensitivity to light) is the major symptom, and uncoordinated movements and mood changes appear together in some cases. Symptoms usually first appear in childhood.