A 6-year-old boy from North India presents with recurrent bone fractures after minor trauma, blue-tinted sclerae, and hearing loss. His mother reports his baby teeth were unusually translucent. Which single feature best distinguishes osteogenesis imperfecta from Marfan syndrome, both of which present with skeletal abnormalities?

Explanation

## Distinguishing Osteogenesis Imperfecta from Marfan Syndrome ### Pathophysiological Basis **Key Point:** OI is caused by **mutations in COL1A1/COL1A2** (type I collagen), affecting bone mineralization and tooth dentin. Marfan syndrome is caused by **FBN1 mutations** (fibrillin-1), affecting connective tissue elasticity and fibrillin-dependent TGF-β signaling. ### Comparative Table | Feature | Osteogenesis Imperfecta | Marfan Syndrome | |---------|------------------------|------------------| | **Gene defect** | COL1A1/COL1A2 (Type I collagen) | FBN1 (Fibrillin-1) | | **Blue sclerae** | Present (classic) | Absent | | **Dentinogenesis imperfecta** | Present (opalescent teeth) | Absent | | **Bone fragility** | Severe, recurrent fractures | Mild skeletal involvement | | **Aortic root dilatation** | Absent | Present (life-threatening) | | **Ectopia lentis** | Absent | Present (upward subluxation) | | **Tall stature** | Short stature (due to fractures) | Tall stature (marfanoid habitus) | | **Arachnodactyly** | Absent | Present | | **Hearing loss** | Present (progressive) | Absent | ### Clinical Pearl **Clinical Pearl:** The **triad of blue sclerae + dentinogenesis imperfecta + recurrent fractures** is pathognomonic for OI and completely absent in Marfan syndrome. Conversely, **aortic root dilatation + ectopia lentis + arachnodactyly** are hallmarks of Marfan syndrome and absent in OI. ### High-Yield Mnemonic **Mnemonic:** **OI = BLUE, Marfan = TALL** - **OI**: Blue sclerae, Dentinogenesis imperfecta, Hearing loss, Bone fragility - **Marfan**: Tall stature, Aortic root dilatation, Lens dislocation (ectopia lentis), Long fingers (arachnodactyly) ### Why the Correct Answer Option B (blue sclerae and dentinogenesis imperfecta) is the **single best discriminator** because: 1. **Both findings are pathognomonic for OI** (type I collagen defect → sclera transparency + abnormal dentin mineralization) 2. **Both are absent in Marfan syndrome** (fibrillin-1 defect does not affect collagen or tooth structure) 3. These findings are **clinically observable** and **specific to OI** [cite:Robbins 10e Ch 7]