What deficiency causes Alkaptonuria?

Alkaptonuria is caused by a deficiency of homogentisic acid oxidase, an enzyme involved in the metabolic pathway of phenylalanine and tyrosine. Normally, this enzyme catalyzes the conversion of homogentisic acid to maleylacetoacetate, and a deficiency leads to the accumulation of homogentisic acid in the body. This accumulation is responsible for the darkening of urine and the potential development of arthritis and other complications due to the deposition of homogentisic acid in connective tissues.

The other options listed do not directly relate to the metabolism affected in alkaptonuria. For instance, arginine decarboxylase is involved in the metabolism of arginine and has no role in the degradation of tyrosine or phenylalanine. Cystathionine synthase is crucial in the metabolism of homocysteine and is associated with cystathionine disorders, not alkaptonuria. Tyrosinase, while important in melanin synthesis and related metabolic pathways, does not directly influence the steps where homogentisic acid is converted into other by-products, which is central to the diagnosis and pathology of alkaptonuria.