**Core Concept**
Lesch-Nyhan syndrome is a rare genetic disorder characterized by excessive uric acid production, resulting from a deficiency of the enzyme hypoxanthine-guanine phosphoribosyltransferase (HGPRT). This enzyme plays a crucial role in purine metabolism, catalyzing the conversion of hypoxanthine and guanine to their respective monophosphate derivatives.

**Why the Correct Answer is Right**
The deficiency of HGPRT leads to the accumulation of hypoxanthine and guanine, which are then broken down into uric acid, resulting in hyperuricemia. This excessive uric acid production causes kidney damage, gouty arthritis, and kidney stones. The lack of HGPRT also disrupts the normal functioning of the purine salvage pathway, leading to the formation of abnormal purine nucleotides.

**Why Each Wrong Option is Incorrect**
**Option A:** Glucose-6-phosphate dehydrogenase (G6PD) deficiency is associated with hemolytic anemia, not Lesch-Nyhan syndrome.
**Option B:** Phenylketonuria (PKU) is caused by a deficiency of phenylalanine hydroxylase, which has no relation to Lesch-Nyhan syndrome.
**Option C:** Cystinuria is a condition characterized by the excessive excretion of cystine in the urine, due to a deficiency of the cystine transporter in the kidneys, not related to Lesch-Nyhan syndrome.

**Clinical Pearl / High-Yield Fact**
Lesch-Nyhan syndrome is characterized by a distinctive clinical triad of excessive uric acid production, intellectual disability, and self-mutilating behavior, such as biting of the lips and fingers.

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