**Core Concept**
Xanthine and hypoxanthine are purine metabolites that play a crucial role in the breakdown of adenosine and guanosine. Their levels are tightly regulated by the enzyme xanthine oxidase, which catalyzes the conversion of hypoxanthine to xanthine and then xanthine to uric acid.

**Why the Correct Answer is Right**
The correct answer is related to a deficiency in the enzyme xanthine oxidase. This enzyme is responsible for the conversion of hypoxanthine to xanthine and then xanthine to uric acid. A deficiency in xanthine oxidase leads to an accumulation of hypoxanthine and xanthine, as they are not being converted to uric acid at the normal rate. This condition is known as xanthinuria, a rare genetic disorder caused by a deficiency in the xanthine oxidase enzyme.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because it does not specify a deficiency in xanthine oxidase or any other enzyme related to purine metabolism.
* **Option B:** This option is incorrect because it does not relate to the accumulation of xanthine and hypoxanthine.
* **Option C:** This option is incorrect because it is unrelated to the metabolism of xanthine and hypoxanthine.

**Clinical Pearl / High-Yield Fact**
Xanthinuria is a rare genetic disorder caused by a deficiency in the xanthine oxidase enzyme. It is characterized by the accumulation of hypoxanthine and xanthine in the urine, leading to kidney stones and other complications.

**Correct Answer:** A.