A 14-month-old girl with recurrent vomiting, developmental delay, and brittle hair that breaks easily is evaluated during a hyperammonemic episode. Plasma shows marked elevation of argininosuccinic acid. Microscopy of hair shafts shows trichorrhexis nodosa. The deficient enzyme is:

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

Explanation

Argininosuccinate lyase cleaves argininosuccinate into fumarate and arginine. Its deficiency causes argininosuccinic aciduria with hyperammonemia, hepatomegaly, and trichorrhexis nodosa from arginine deficiency impairing hair keratin cross-linking. Citrulline is also elevated because the preceding step is intact. The key discriminator against argininosuccinate synthetase deficiency (citrullinemia type I) is the presence of accumulated argininosuccinate itself, which cannot form if synthetase is absent.

Reference: Nelson Textbook of Pediatrics, 21st ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.