An 8-year-old tall, thin boy with arachnodactyly, high-arched palate, and pectus excavatum is being differentiated from Marfan syndrome. Slit-lamp examination shows superior and temporal subluxation of the lens. Urine screening is positive for homocystine. What is the diagnosis?

• A Marfan syndrome due to fibrillin-1 mutation
• B Homocystinuria due to cystathionine beta-synthase deficiency ✓
• C Ehlers-Danlos syndrome, kyphoscoliotic type
• D Classical homocystinuria due to methylenetetrahydrofolate reductase deficiency

Explanation

Classical homocystinuria is caused by cystathionine beta-synthase (CBS) deficiency leading to elevated homocysteine and methionine. Skeletal features mimic Marfan syndrome, but the direction of lens dislocation differs: CBS deficiency produces inferior and nasal subluxation classically, while exam texts emphasise downward dislocation versus the upward superotemporal dislocation of Marfan. The positive urine homocystine test confirms CBS deficiency over fibrillin defect.

Reference: Nelson Textbook of Pediatrics, 21st ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.