**Question:** Lesch Nyhan syndrome is caused by:

**Core Concept:** Lesch Nyhan syndrome is a rare genetic disorder that affects the purine catabolism pathway, specifically the hypoxanthine-guanine phosphoribosyl-transerase (HPRT) enzyme deficiency.

**Why the Correct Answer is Right:** Lesch Nyhan syndrome is caused by a deficiency in the hypoxanthine-guanine phosphoribosyl-transerase (HPRT) enzyme. HPRT is essential for maintaining purine homeostasis in the body. In the absence of HPRT, an excess of hypoxanthine and guanine accumulate, leading to the development of the disorder.

**Why Each Wrong Option is Incorrect:**

A. This option is incorrect because it does not mention the primary cause of Lesch Nyhan syndrome, which is the deficiency in the HPRT enzyme.
B. This option is incorrect because it does not address the specific enzyme deficiency causing Lesch Nyhan syndrome.
C. This option is incorrect because it focuses on a different genetic disorder (Fanconi anaemia) and does not address the HPRT enzyme deficiency.
D. This option is incorrect because it is a treatment method, not the cause of Lesch Nyhan syndrome.

**Clinical Pearl / High-Yield Fact:** Lesch Nyhan syndrome is a prime example of the importance of proper purine catabolism and the role of HPRT enzyme in maintaining homeostasis. Inadequate enzyme function can lead to severe neurological and behavioral symptoms, emphasizing the need for proper genetic counseling and disease awareness.