A screened newborn shows elevated citrulline and argininosuccinic acid on tandem mass spectrometry. At 6 months she has brittle, tufted, friable hair that breaks with minimal traction. Microscopy of the hair shaft is diagnostic. The underlying enzyme deficiency is:

• A Argininosuccinate synthetase
• B Argininosuccinate lyase ✓
• C Ornithine transcarbamoylase
• D Arginase

Explanation

Argininosuccinate lyase deficiency produces argininosuccinic aciduria, characterized by hypercitrullinemia with accumulation of argininosuccinate, and trichorrhexis nodosa, the brittle node-like hair caused by argininosuccinate interfering with hair keratin. In citrullinemia type I (synthetase deficiency) argininosuccinate is absent, which excludes option A. OTC deficiency gives low citrulline with orotic aciduria, and arginase deficiency gives hyperargininemia without hair changes.

Reference: Nelson Textbook of Pediatrics, 21st ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.