Alkaptonuria is caused by defect in which of the following enzymes?
**Core Concept**
Alkaptonuria is a rare genetic disorder characterized by the accumulation of homogentisic acid in the body, leading to dark urine, arthritis, and other systemic complications. It is an example of a metabolic disorder resulting from an enzyme deficiency.
**Why the Correct Answer is Right**
The correct answer is Homogentisate oxidase because it is the enzyme responsible for converting homogentisic acid to 4-maleylacetoacetic acid in the tyrosine catabolic pathway. In alkaptonuria, the deficiency of homogentisate oxidase leads to the accumulation of homogentisic acid, which is then oxidized to benzoquinone acetic acid, causing the characteristic symptoms of the disease. This enzyme is crucial for the breakdown of tyrosine, and its deficiency results in the accumulation of toxic metabolites.
**Why Each Wrong Option is Incorrect**
**Option A:** Enolase is an enzyme involved in glycolysis, converting 2-phosphoglycerate to enolpyruvate. It is not related to the tyrosine catabolic pathway or alkaptonuria.
**Option C:** Pyruvate carboxylase is an enzyme involved in gluconeogenesis, converting pyruvate to oxaloacetate. It is not related to the tyrosine catabolic pathway or alkaptonuria.
**Option D:** None of the above is incorrect because one of the options above (Homogentisate oxidase) is indeed the correct answer.
**Clinical Pearl / High-Yield Fact**
Alkaptonuria is an autosomal recessive disorder, meaning that an individual must inherit two defective copies of the gene (one from each parent) to express the disease. It is essential to recognize this disorder as it can lead to significant morbidity and mortality if left untreated.
**✓ Correct Answer: B. Homogentisate oxidase**