A 16-year-old tall, thin boy has long limbs and mild pectus excavatum. Unlike his cousin with Marfan syndrome, he has intellectual disability, livedo reticularis of the legs, and lens dislocation directed downward and inward. He had a deep vein thrombosis last year. Which biochemical finding is expected?

• A Elevated phenylalanine with normal tyrosine
• B Deficiency of alpha-galactosidase A
• C Elevated plasma homocysteine and homocystine in urine ✓
• D Accumulation of glycosaminoglycans in leukocytes

Explanation

Homocystinuria due to cystathionine beta-synthase deficiency produces a marfanoid habitus that closely imitates Marfan syndrome, but three features discriminate it: inferior and nasal lens subluxation instead of superior temporal, thromboembolic events, and intellectual disability. The diagnosis rests on elevated plasma total homocysteine and homocystine excretion in urine, with many patients responding biochemically to high-dose pyridoxine. Alpha-galactosidase C deficiency causes Fabry disease, unrelated to this picture.

Reference: Robbins and Cotran Pathologic Basis of Disease, 10th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.