A three and half-year-old male child, who presented with characteristic self-mutilating behavior. He had history of developmental delay, difficulty in social interaction, attention deficit and features of autism. His serum blood biochemistry was normal except for low hemoglobin levels and raised serum uric acid levels. Which of the following enzyme is deficient?

**Core Concept**
The child's presentation with self-mutilating behavior, developmental delay, social interaction difficulties, attention deficit, and features of autism, along with normal serum blood biochemistry except for low hemoglobin levels and raised serum uric acid levels, points towards a disorder of purine metabolism.

**Why the Correct Answer is Right**
The child's symptoms are characteristic of Lesch-Nyhan syndrome, a disorder caused by 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 ribonucleotides. The deficiency of HGPRT leads to the accumulation of uric acid, resulting in hyperuricemia and subsequent renal damage. Additionally, the enzyme deficiency leads to the breakdown of purine nucleotides, resulting in the formation of uric acid and the release of xanthine and hypoxanthine, which are then converted to uric acid, further exacerbating the condition.

**Why Each Wrong Option is Incorrect**
* **Option A:** Glucose-6-phosphate dehydrogenase (G6PD) deficiency is a disorder that affects the enzyme responsible for catalyzing the conversion of glucose-6-phosphate to 6-phosphogluconolactone in the pentose phosphate pathway. It does not account for the child's presentation.
* **Option B:** Phenylketonuria (PKU) is a disorder caused by a deficiency of the enzyme phenylalanine hydroxylase, leading to the accumulation of phenylalanine in the body. It does not directly relate to the child's symptoms.
* **Option C:** Galactosemia is a disorder caused by a deficiency of the enzyme galactose-1-phosphate uridyltransferase, leading to the accumulation of galactose-1-phosphate in the body. It does not account for the child's presentation.

**Clinical Pearl / High-Yield Fact**
Lesch-Nyhan syndrome is a classic example of a disorder of purine metabolism, and HGPRT deficiency is a key enzyme deficiency. It is essential to recognize the symptoms of Lesch-Nyhan syndrome, including self-mutilating behavior, developmental delay, and social interaction difficulties, to diagnose the condition accurately.

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

✓ Correct Answer: C. HGP