A 4-year-old boy presents with self-mutilating behaviour, hyperuricaemia, and choreoathetosis. His urine shows orange crystals. The enzyme deficient in this condition is:

• A Adenosine deaminase (ADA)
• B Xanthine oxidase
• C Purine nucleoside phosphorylase (PNP)
• D Hypoxanthine-guanine phosphoribosyltransferase (HGPRT) ✓

Explanation

Lesch-Nyhan syndrome is caused by complete deficiency of HGPRT, the key salvage pathway enzyme for hypoxanthine and guanine. Without salvage, purines are catabolised to uric acid, causing hyperuricaemia; the mechanism of neurological damage involves dopamine system dysfunction in the basal ganglia. Orange urate crystals in urine are a hallmark. ADA deficiency causes SCID (immunodeficiency), not hyperuricaemia.

Reference: Harper's Illustrated Biochemistry, 32nd ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.