A 28-year-old presents with marfanoid habitus, downward lens dislocation, intellectual disability, and thromboembolism. Urinary nitroprusside test is positive. The enzyme deficiency responsible is:
- A Phenylalanine hydroxylase
- B Cystathionine beta-synthase ✓
- C Homogentisate oxidase
- D Branched-chain alpha-keto acid dehydrogenase
Correct answer: B. Cystathionine beta-synthase
Explanation
Homocystinuria due to cystathionine beta-synthase deficiency causes accumulation of homocysteine, leading to Marfan-like features (tall, arachnodactyly) but with DOWNWARD lens dislocation (cf. upward in Marfan), intellectual disability, and thromboembolism due to homocysteine-induced endothelial injury. The nitroprusside (cyanide-nitroprusside) test detects sulfhydryl-containing amino acids including homocysteine in urine. PAH deficiency causes PKU; homogentisate oxidase causes alkaptonuria; BCKDH causes maple syrup urine disease.
Reference: Robbins & Cotran Pathologic Basis of Disease, 10th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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