1. A congenital metabolic disorder characterized by aminoaciduria, a pellagralike, light-sensitive skin rash, and a temporary cerebellar ataxia. Also called Hartnup syndrome, H disease. 2. An inherited metabolic disease that is caused by abnormalities of the renal tubules and is characterized especially by aminoaciduria involving only monocarboxylic monoamines, a dry red scaly rash, and episodic muscular incoordination due to the effects of the disease on the cerebellum.