A 12 year old child presents with mental retardation and history of self mutilation. Which of the following tests will help in determining the diagnosis

D i.e. Serum Uric Acid LevelsLesch - Nyhan syndrome is an X- linked, recessive disorder associated with a complete deficiency of hypoxanthine guanine phosphoribosyl transferase (HGP)Q and therefore has the inability to salvage hypoxanthine or guanine, from which excessive uric acid are produced. They are poorly coordinated, mentally retarded, extremely hostile male children who show compulsive self aggressive and self destructive tendencies; they mutilate themselves by biting off their fingers, toes and lipsQ. The features of hyper ureamia & gout are also presented.Uric acid (urate) levels in urine and plasma (serum) is the key initial investigation in purine metabolism disorder (like Lesch Nyhan syndrome) and should be measured to avoid missing the diagnosisQ.