**Core Concept**
The question is testing the understanding of purine metabolism, specifically the enzyme responsible for converting hypoxanthine and xanthine to uric acid. This involves the xanthine oxidase pathway, where xanthine is converted to uric acid through the action of xanthine oxidase.

**Why the Correct Answer is Right**
Xanthine oxidase is a key enzyme in the catabolism of purines. It catalyzes the conversion of hypoxanthine to xanthine and then xanthine to uric acid. In the absence of this enzyme, xanthine and hypoxanthine accumulate in the blood, leading to hypouricemia. This is because uric acid, the end product of purine catabolism, is not formed in sufficient quantities. Xanthine oxidase utilizes FAD, molybdenum, and iron as cofactors and is inhibited by allopurinol.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not specify the enzyme responsible for purine catabolism. While it mentions an enzyme involved in purine metabolism, it does not directly relate to the accumulation of hypoxanthine and xanthine.

**Option B:** This option is incorrect because it refers to an enzyme involved in the synthesis of purines, not their catabolism. Adenine phosphoribosyltransferase (APRT) is involved in the conversion of adenine to AMP.

**Option C:** This option is incorrect because it refers to an enzyme involved in the regulation of purine synthesis, not catabolism. HGPRT (Hypoxanthine-Guanine Phosphoribosyltransferase) is involved in the salvage pathway of purine synthesis.

**Option D:** This option is incorrect because it is the correct answer. Xanthine oxidase is the enzyme deficient in this condition, leading to the accumulation of hypoxanthine and xanthine and hypouricemia.

**Clinical Pearl / High-Yield Fact**
Allopurinol, a xanthine oxidase inhibitor, is often used to treat conditions characterized by excessive uric acid production, such as gout. It works by inhibiting the conversion of hypoxanthine to xanthine and xanthine to uric acid.

**Correct Answer:** D. Xanthine oxidase.