A 10 year old child with aggressive, self mutilating behavior and poor concentration is brought with presenting complaints of joint pain and reduced urinary output. Child's presentation is most likely due to the deficiency of which enzyme?

The child in the question stem is showing features of Lesch nyhan syndrome caused by deficiency of HGPase. Hypoxanthine guanine phosphoribosyl transferase is an enzyme responsible for the formation of hypoxanthine to inosoic acid and guanine to guanylic acid. This enzyme deficiency can present in a complete form only in boys and is transmitted through sex linked mode of inheritance. Leschnyhan syndrome is a severe form of this enzyme deficiency resulting in mental retardation, compulsive mutilating behavior and choreoathetosis. In addition to the above, patients exhibit hyperuricemia and hyperuricosuria as well as signs of gouty ahritis and uric acid stone disease. Treatment consist of dietary reduction of purines, increased fluid uptake, alkalinization of urine to pH 6.6 with potassium citrate. Ref: Clinical Pediatric Urology By A. Barry Belman, Page 1231;Inherited Metabolic Diseases: A Guide to 100 Conditions By Steve Hannigan, Page 85; Harrison's Principle of Internal Medicine, 18th Edition, Chapters 359, 361