Melanuria (black urine) is caused by abnormal catabolism of

Options

A. Alanine
B. Tyrosine
C. Proline
D. Tryptophan

Answer

**Step-by-Step Solution:**

1. **Understanding Melanuria**: Melanuria is a condition characterized by the presence of black urine. It is important to identify the biochemical pathway that leads to this condition.

2. **Identifying the Cause**: Melanuria is primarily caused by a genetic mutation that affects the catabolism of certain amino acids. The specific mutation is in the HGD gene, which is responsible for the breakdown of tyrosine and phenylalanine.

3. **Biochemical Pathway**: During the normal catabolism of phenylalanine and tyrosine, an intermediate product called homogentisic acid is formed. In individuals with melanuria, there is an impairment in the breakdown of this acid.

4. **Effect of Homogentisic Acid**: When there is an excess of homogentisic acid in the body, it gets excreted in the urine. When this urine is exposed to air, the homogentisic acid oxidizes, resulting in a dark or black coloration of the urine.

5. **Evaluating the Options**:
- **Alanine**: Not involved in the pathway leading to melanuria.
- **Tyrosine**: Correct answer, as it is directly related to the condition.
- **Proline**: Not involved in the pathway leading to melanuria.
- **Tryptophan**: Not involved in the pathway leading to melanuria.

6. **Conclusion**: Based on the analysis, the amino acid that is abnormally catabolized in the case of melanuria is **tyrosine**.

**Final Answer**: Tyrosine (Option B).

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