Homocystinuria due to cystathionine beta-synthase (CBS) deficiency is characterised by all of the following EXCEPT:
- A Ectopia lentis (downward lens dislocation)
- B Thromboembolic events due to endothelial damage
- C Normal plasma methionine with elevated cysteine ✓
- D Intellectual disability and Marfanoid habitus
Correct answer: C. Normal plasma methionine with elevated cysteine
Explanation
CBS deficiency impairs the transsulfuration of homocysteine to cystathionine, causing accumulation of homocysteine and its precursor methionine — so plasma methionine is ELEVATED, not normal, and cysteine is REDUCED (not elevated) because its biosynthesis from homocysteine is blocked. Clinical features include ectopia lentis (inferior displacement, unlike Marfan's superior), Marfanoid habitus, intellectual disability, and thromboembolic disease. Treatment: pyridoxine (B6) in responsive cases, methionine restriction.
Reference: Harper's Illustrated Biochemistry, 32nd ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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