A 12-year-old boy evaluated for a stroke at this young age is found to be tall with long limbs, arachnodactyly, pectus excavatum, and genu valgum. He has intellectual disability and livedo reticularis. On slit-lamp examination, the lens is displaced downward and nasally. Plasma total homocysteine is markedly elevated. The underlying enzyme deficiency is:

• A Cystathionine beta-synthase ✓
• B Alpha-L-iduronidase
• C Lysyl hydroxylase
• D Methylenetetrahydrofolate reductase

Explanation

Classic homocystinuria is due to cystathionine beta-synthase deficiency. It mimics Marfan syndrome (tall habitus, arachnodactyly, pectus deformity) but adds intellectual disability, thromboembolic events, and an INFERIOR AND NASAL lens subluxation, whereas Marfan lens displacement is superotemporal. Lysyl hydroxylase deficiency causes kyphoscoliotic type Ehlers-Danlos, alpha-L-iduronidase causes Hurler syndrome, and MTHFR variants rarely produce this classic phenotype.

Reference: Nelson Textbook of Pediatrics, 21st ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.