Defective purine salvage due to absent HGPRT, which converts hypoxanthine to IMP and guanine to GMP. (Guanine, Hypoxanthine → Xanthine → Uric acid → Urine)

- Results in excess uric acid production and de novo purine synthesis

- X-linked recessive

Findings: intellectual disability, self-mutilation, aggression, hyperuricemia (orange "sand" in diaper), gout, dystonia)

Treatment: allopurinol or febuxostat