The main symptom of brownish-yellow disease arthritis is that the whole body skin, sclera and cornea pigmentation is brownish-yellow, the ear, nose and cartilage can become blue, the edge of the tympanic membrane is grayish-black, and the hearing is often reduced. Due to the lack of uronic acid oxidase, the intermediate metabolites of phenylalanine and tyrosine (uronic acid) cannot be further oxidized and decomposed, and accumulate in the body. This causes the skin, sclera and cartilage to become darker, while uronic acid causes hyperpigmentation of cartilage and other connective tissues, and degenerative arthritis of the spine and peripheral large joints. On the other hand, urinary black acid is excreted in the urine, where it is alkalized and oxidized, causing the urine to turn black, so it is also called black acid urine (Alkaptonuria) disease. This disease is a rare genetic disease and is rare. Pathogenesis Alkaptonuria is a rare hereditary disorder of amino acid metabolism. Usually, there is a family history of the disease, and the incidence varies between males and females and is approximately 2:1. Fuchsinosis arthritis is caused by pigmentation of the intervertebral discs or cartilage, resulting in degenerative disc degeneration and arthrosis, i.e., fuchsinosis-like pigmentation. Deposition in the structural tissues of the joints causes xanthogranulomatous arthritis. Pathogenesis The metabolic pathway that converts phenylalanine and tyrosine to acetoacetic acid, with uronic acid (HGA or 2,5-dihydroxyphenylacetic acid) being the last compound in this metabolic pathway to contain an intact aromatic ring. The enzyme that catalyzes the cleavage of the aromatic ring, i.e., uronic acid oxidase, is normally present in the soluble fraction of liver and kidney tissue. This enzyme is highly specific for the breakdown of uronic acid and is not contained in tissues other than the liver and kidney. In patients with this disease, there is a complete lack of this enzyme activity in the liver and kidneys, which can result in the non-catabolism of uronic acid into acetoacetic acid and jenhoic acid, so that other metabolic mechanisms are required to process uronic acid. The kidneys have a high clearance of urea, the renal tubules actively secrete urea, and once the kidneys excrete it, it is gradually oxidized to form polymers that can cause urine discoloration. The mechanism of urea deposition in tissues causing browning is not well understood. Uric acid has a tendency to be deposited in skin and cartilage, where it can be bound by physical gravity. In addition, the breakdown products of uronic acid can irreversibly bind to connective tissue and form polymers that can cause hyperpigmentation. The tissues become discolored and fragile and may even fracture, leading to degenerative lesions of the intervertebral discs and joints. In addition, uronic acid can act directly on collagen synthesis by inhibiting lysyl hydroxylase. Pathology Darkening of connective tissue and cartilage is the basic pathology of the disease. Uric acid is deposited in the skin, cornea, cartilage, tendons, ligaments, endothelium, endocardium, thyroid, lungs, kidneys and other organs, resulting in darkening of these organ tissues, and involvement of cartilage may lead to subchondral bone exfoliation.