A patient reports a change in colour of urine on air exposure. All are true about the condition shown below except:
## Core Concept
The condition described, where a patient's urine changes color on air exposure, is suggestive of **alkaptonuria**. Alkaptonuria is a rare genetic disorder caused by a deficiency of the enzyme **homogentisate 1,2-dioxygenase**, which is involved in the breakdown of the amino acids **tyrosine** and **phenylalanine**.
## Why the Correct Answer is Right
In alkaptonuria, the deficiency of homogentisate 1,2-dioxygenase leads to the accumulation of **homogentisic acid** in the body. Homogentisic acid is then excreted in the urine. Upon exposure to air (oxygen), homogentisic acid undergoes **oxidation**, which leads to the formation of **benzoquinone acetic acid** and ultimately results in the urine turning **dark** or **black**. This characteristic change in urine color is a diagnostic clue for alkaptonuria.
## Why Each Wrong Option is Incorrect
- **Option A:** Without the specific details of option A, we cannot directly address its accuracy. However, if it pertains to a characteristic feature or consequence of alkaptonuria, its correctness would depend on its alignment with known facts about the condition.
- **Option B:** Similarly, without specifics, we can't directly refute or confirm. If it's a true statement about alkaptonuria, then it's not the correct answer to the question asking for an exception.
- **Option C:** Again, lacking details, but if it accurately describes an aspect of alkaptonuria, it wouldn't be the incorrect statement we're looking for.
- **Option D:** This is marked as the correct answer, implying that options A, B, or C must contain incorrect statements about alkaptonuria.
## Clinical Pearl / High-Yield Fact
A key clinical feature of alkaptonuria, aside from the urine changing color upon standing, is the development of **ochronosis**. Ochronosis refers to the bluish-black pigmentation of connective tissues, such as **cartilage**, **tendons**, and **skin**, due to the accumulation of homogentisic acid and its oxidation products. This pigmentation can lead to a range of clinical manifestations, including **joint problems** and **cardiovascular issues**.
## Correct Answer: D.