An 18-year-old tall, thin young man with pectus excavatum and long slender fingers is being evaluated for possible connective tissue disease. He has upward and temporal subluxation of the lenses, but unlike a previously evaluated cousin with a similar body habitus, he has no ectopia lentis history of thrombosis, and plasma homocysteine levels are normal. The diagnosis and defective protein are:
- A Marfan syndrome, fibrillin-1 ✓
- B Homocystinuria, cystathionine beta-synthase
- C Ehlers-Danlos syndrome vascular type, type III procollagen
- D Homocystinuria, methylenetetrahydrofolate reductase
Explanation
Both Marfan syndrome and homocystinuria produce tall stature, long limbs, arachnodactyly, and lens dislocation, making this a classic discrimination. The direction of lens displacement separates them: fibrillin-1 deficiency weakens the zonular fibers, allowing superior-temporal subluxation, while cystathionine beta-synthase deficiency in homocystinuria displaces the lens inferior-nasally. Normal plasma homocysteine excludes homocystinuria, eliminating options B and D regardless of direction. Vascular-type Ehlers-Danlos features arterial rupture and translucent skin rather than lens dislocation, ruling out option C.
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.