A tall 14-year-old boy evaluated for marfanoid habitus is found to have subluxation of the lenses directed downward and inward, mild intellectual disability, and a previous deep vein thrombosis. Plasma homocysteine is markedly elevated. Deficiency of which enzyme explains these findings?

• A Alpha-L-iduronidase
• B Homogentisic acid oxidase
• C Methylenetetrahydrofolate reductase
• D Cystathionine beta-synthase ✓

Explanation

Classical homocystinuria is caused by cystathionine beta-synthase deficiency, producing ectopia lentis directed downward and inward, marfanoid habitus, intellectual disability, osteoporosis, and thromboembolic events. The direction of lens dislocation kills the Marfan distractor: in Marfan syndrome the lens subluxes upward and outward. Homogentisic acid oxidase deficiency causes alkaptonuria, and alpha-L-iduronidase deficiency causes Hurler syndrome.

Reference: Nelson Textbook of Pediatrics, 21st ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.