A 16-year-old boy evaluated for intellectual disability has long thin limbs, high-arched feet, and pectus carinatum resembling Marfan syndrome. However, the lenses are displaced downward and inward in both eyes. He has a history of a deep venous thrombosis last year. Which enzyme deficiency is the most likely cause?

• A Cystathionine beta-synthase ✓
• B Alpha-L-iduronidase
• C Fibrillin-1
• D Procollagen lysyl hydroxylase

Explanation

Classical homocystinuria from cystathionine beta-synthase deficiency produces a marfanoid habitus, intellectual disability, osteoporosis, and a marked tendency to arterial and venous thromboembolism. The discriminating sign is the direction of ectopia lentis: inferior and nasal displacement in homocystinuria versus upward displacement in Marfan syndrome. Alpha-L-iduronidase causes Hurler disease, fibrillin-1 is defective in Marfan, and lysyl hydroxylase deficiency causes EDS kyphoscoliotic type.

Reference: Nelson Textbook of Pediatrics, 21st ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.