USMLE Step 1 · Biochemistry & Genetics

Marfan syndrome and homocystinuria vs Ehlers-Danlos and Loeys-Dietz syndromes

How to tell Marfan syndrome and homocystinuria and Ehlers-Danlos and Loeys-Dietz syndromes apart when a board question makes them look alike.

Fastest separator

Marfan is lens up/out and aortic root; homocystinuria is lens down/in and thrombosis; EDS is skin/joint collagen; Loeys-Dietz is bifid uvula and tortuous aneurysms.

Side by side

Marfan syndrome and homocystinuriaEhlers-Danlos and Loeys-Dietz syndromes

Key clue

Marfan causes tall habitus, lens up/out, aortic root dilation, and fibrillin-1 defect; homocystinuria causes marfanoid habitus, lens down/in, thrombosis, and intellectual disability.

Ehlers-Danlos causes hyperextensible skin, joint hypermobility, easy bruising, and collagen defects; Loeys-Dietz causes hypertelorism, bifid uvula, arterial tortuosity, and aggressive aneurysms.

Mechanism

FBN1 mutation weakens microfibrils and increases TGF-beta signaling; homocystinuria disrupts methionine metabolism and connective tissue/cardiovascular risk.

Collagen synthesis/processing defects cause Ehlers-Danlos, while TGF-beta receptor pathway mutations cause Loeys-Dietz vascular phenotype.

Labs and tests

Aortic root dilation with upward lens dislocation supports Marfan; thrombosis with downward lens dislocation supports homocystinuria.

Hyperelastic skin/joints supports Ehlers-Danlos; bifid uvula and arterial tortuosity support Loeys-Dietz.

The trap

Marfanoid body habitus alone is not enough; lens direction, thrombosis, skin elasticity, and bifid uvula decide.

The repair move

For connective tissue syndromes, attach habitus to lens direction, vascular pattern, and skin/joint findings.

Try a free question

Core

A 19-year-old tall thin man with arm span exceeding height (>1.05 ratio), long arachnodactyly fingers, pectus excavatum, high-arched palate, upward and outward lens subluxation, mitral valve prolapse, and dilation of the aortic root on echocardiogram presents to clinic. Family history of similar features is present. Which condition is most likely?

Show answer and explanation

Answer: A. Marfan syndrome (FBN1 mutation). Marfan syndrome is autosomal dominant FBN1 (fibrillin-1) mutation with abnormal microfibrils and increased TGF-beta signaling. Features: tall stature with arachnodactyly, upward/outward lens subluxation, aortic root dilation/dissection, mitral valve prolapse, joint laxity, scoliosis, pectus deformity.

Why the other choices are tempting

B. Homocystinuria has DOWNWARD/INWARD lens subluxation, marfanoid habitus, thrombosis, and intellectual disability, NOT upward/outward lens with aortic root dilation alone.
C. Ehlers-Danlos syndrome has joint hypermobility, hyperextensible skin, and easy bruising, not lens subluxation with aortic root dilation as the primary picture.
D. Loeys-Dietz syndrome has hypertelorism, bifid uvula, cleft palate, arterial tortuosity, and aggressive aneurysms throughout the arterial tree.
E. Achondroplasia has short stature with FGFR3 mutation, the opposite of marfanoid tall habitus.

Flashcards

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What clue supports Marfan syndrome and homocystinuria?

Choose Marfan syndrome and homocystinuria when the stem gives this pattern: Marfan causes tall habitus, lens up/out, aortic root dilation, and fibrillin-1 defect; homocystinuria causes marfanoid habitus, lens down/in, thrombosis, and intellectual disability.

What clue supports Ehlers-Danlos and Loeys-Dietz syndromes?

Choose Ehlers-Danlos and Loeys-Dietz syndromes when the stem gives this contrasting pattern: Ehlers-Danlos causes hyperextensible skin, joint hypermobility, easy bruising, and collagen defects; Loeys-Dietz causes hypertelorism, bifid uvula, arterial tortuosity, and aggressive aneurysms.

Quick answers

What is the fastest way to tell Marfan syndrome and homocystinuria from Ehlers-Danlos and Loeys-Dietz syndromes?