**Core Concept**
Lesch Nyhan syndrome is a rare genetic disorder caused by a deficiency of the enzyme hypoxanthine-guanine phosphoribosyltransferase (HGPRT). This enzyme is crucial for the purine metabolism pathway, allowing cells to recycle purines by converting hypoxanthine and guanine into inosine monophosphate (IMP). Without sufficient HGPRT, cells accumulate toxic purine metabolites, leading to various clinical manifestations.

**Why the Correct Answer is Right**
The correct answer choice is related to the clinical features of Lesch Nyhan syndrome. Patients with this condition often exhibit self-mutilating behaviors, such as biting their lips, tongue, or fingers, due to the accumulation of uric acid and other purine metabolites in the nervous system. This leads to severe neurological damage, including intellectual disability, spasticity, and choreoathetosis. The deficiency of HGPRT results in the inability to recycle purines, causing an overproduction of uric acid, which contributes to the development of kidney stones and gout.

**Why Each Wrong Option is Incorrect**
**Option A:** This choice is incorrect because it does not accurately describe the primary clinical features of Lesch Nyhan syndrome. While patients may exhibit some behavioral problems, self-mutilation is a hallmark symptom of this condition.

**Option B:** This choice is incorrect because it is a description of a different genetic disorder, Fragile X syndrome, which is caused by a deficiency of the FMR1 gene. Although both conditions can cause intellectual disability, they are distinct and have different underlying mechanisms.

**Option C:** This choice is incorrect because it is a description of another genetic disorder, Fabry disease, which is caused by a deficiency of the enzyme alpha-galactosidase A. This condition is characterized by the accumulation of globotriaosylceramide in the body, leading to various systemic symptoms.

**Option D:** This choice is incorrect because it does not accurately describe the primary genetic defect causing Lesch Nyhan syndrome. While there are other genetic disorders that can cause intellectual disability, Lesch Nyhan syndrome is a distinct entity caused by a deficiency of the HGPRT enzyme.

**Clinical Pearl / High-Yield Fact**
A key clinical feature of Lesch Nyhan syndrome is the presence of elevated uric acid levels in the blood and urine, which can lead to the development of kidney stones and gout. This can be a useful diagnostic clue for clinicians when evaluating patients with self-mutilating behaviors and intellectual disability.

**Correct Answer:** D. Lesch Nyhan syndrome is caused by a deficiency of the enzyme hypoxanthine-guanine phosphoribosyltransferase (HGPRT).