**Core Concept**
Purine disorders refer to a group of rare genetic conditions characterized by the accumulation of purine nucleotides or their breakdown products in the body. This leads to various clinical manifestations, including kidney stones, gout, and neurological symptoms. The primary defect in these disorders involves the deficiency or malfunction of enzymes involved in purine metabolism.

**Why the Correct Answer is Right**
The most common disorder of purine metabolism is Lesch-Nyhan syndrome, which is caused by a deficiency of the enzyme hypoxanthine-guanine phosphoribosyltransferase (HGPRT). This enzyme is crucial for the conversion of hypoxanthine and guanine to their respective monophosphate derivatives, thus participating in the salvage pathway of purine nucleotide synthesis. The deficiency of HGPRT leads to the accumulation of uric acid and its precursors, resulting in severe clinical symptoms.

**Why Each Wrong Option is Incorrect**
**Option A:** Although xanthinuria is a disorder of purine metabolism, it is not the most common disorder. Xanthinuria is caused by a deficiency of xanthine oxidase, an enzyme involved in the catabolism of purines.
**Option B:** Gout is a condition characterized by the deposition of monosodium urate crystals in the joints, but it is not a disorder of purine metabolism per se. Gout can be caused by various factors, including genetic predisposition, diet, and certain medical conditions.
**Option C:** Hypoxanthine-guanine phosphoribosyltransferase (HGPRT) deficiency is the correct answer, but it is not listed as an option. This enzyme is crucial for the salvage pathway of purine nucleotide synthesis.

**Clinical Pearl / High-Yield Fact**
Lesch-Nyhan syndrome is a classic example of a genetic disorder that affects multiple systems in the body. It highlights the importance of purine metabolism in maintaining normal physiological functions and the consequences of its disruption.

**Correct Answer: D. Hypoxanthine-guanine phosphoribosyltransferase (HGPRT) deficiency.**