A baby that was apparently normal at bih begins to show a delay in motor development by 3 months of age. At one year of age, the child begins to develop spasticity and writhing movements. At age three, compulsive biting of fingers and lips and head-banging appear. At pubey, the child develops ahritis, and death from renal failure occurs at age 25. This patient's condition is due to an enzyme deficiency in which of the following biochemical pathways?

The patient has a classical case of Lesch-Nyhan syndrome, an X-linked disorder due to severe deficiency of the purine salvage enzyme hypoxanthine-guanine phosphoribosyl transferase (HP). This defect is associated with excessive de novo purine synthesis, hyperuricemia, and the clinical signs and symptoms described in the question stem. The biochemical basis of the often striking self-mutilatory behavior (which may require restraints and even tooth extraction) has never been established. Treatment with allopurinol inhibits xanthine oxidase and reduces gouty ahritis, urate stone formation, and urate nephropathy. It does not, however, modify the neurologic/psychiatric presentation.