A 1-year-old presents with recurrent vomiting, lethargy after high-protein feeds, and brittle hair that fractures easily. Microscopy of hair shows trichorrhexis nodosa. Plasma ammonia is elevated and plasma amino acid analysis shows a large peak of argininosuccinic acid. Which enzyme is deficient?
- A Argininosuccinate synthetase
- B Argininosuccinate lyase ✓
- C Arginase
- D Ornithine transcarbamylase
Correct answer: B. Argininosuccinate lyase
Explanation
Accumulation of argininosuccinate localizes the block to argininosuccinate lyase, which cleaves argininosuccinate into arginine and fumarate. Trichorrhexis nodosa from argininosuccinic acid interfering with hair keratin is the classic clue. In synthetase deficiency (citrullinemia type I) the accumulated metabolite is citrulline, not argininosuccinate, which kills option A.
Reference: Nelson Textbook of Pediatrics, 21st ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.