**Core Concept**
Hypoxanthine-guanine phosphoribosyl transferase (HGPRT) is an enzyme crucial for purine metabolism, catalyzing the conversion of hypoxanthine and guanine to their respective monophosphate derivatives, thereby participating in the salvage pathway of purine nucleotide synthesis. This enzyme plays a vital role in maintaining intracellular purine homeostasis.

**Why the Correct Answer is Right**
The correct answer, **C. PRPP is the substrate required**, is incorrect because HGPRT actually utilizes phosphoribosyl pyrophosphate (PRPP) as a co-substrate, not a substrate. In the reaction catalyzed by HGPRT, PRPP is converted into phosphoribosyl pyrophosphate, which then reacts with hypoxanthine or guanine to form their respective monophosphate derivatives. The correct substrates for the HGPRT reaction are hypoxanthine and guanine.

**Why Each Wrong Option is Incorrect**
**Option A:** HGPRT is predominantly present in the liver, kidneys, and brain, not just the liver.
**Option B:** This statement is true; HGPRT is indeed involved in purine metabolism, specifically in the salvage pathway.
**Option D:** This statement is also true; a deficiency in HGPRT leads to Lesch-Nyhan disease, a genetic disorder characterized by excessive uric acid production, neurological dysfunction, and self-mutilating behaviors.

**Clinical Pearl / High-Yield Fact**
Lesch-Nyhan disease is an X-linked recessive disorder caused by a mutation in the HGPRT gene, leading to a deficiency of the enzyme. Understanding the role of HGPRT in purine metabolism and its clinical implications is essential for diagnosing and managing patients with this condition.

**✓ Correct Answer: C. PRPP is the substrate required**