MSH: An inborn error of amino acid metabolism resulting from a defect in the enzyme HOMOGENTISATE 1,2-DIOXYGENASE, an enzyme involved in the breakdown of PHENYLALANINE and TYROSINE. It is characterized by accumulation of HOMOGENTISIC ACID in the urine, OCHRONOSIS in various tissues, and ARTHRITIS.,NCI: A rare autosomal recessive disorder characterized by abnormalities in the metabolism of phenylalanine and tyrosine. It results in the accumulation in the blood of homogentisic acid which is excreted in the urine. The presence of homogentisic acid in the urine causes its color to turn black. The excessive amount of homogentisic acid in the blood may cause damage to cartilage and heart valves, and may result in the formation of kidney stones.