A 28-year-old woman from rural Maharashtra presents with a 3-year history of progressive joint pain, skin hyperextensibility, and easy bruising. On examination, she has hypermobile joints (positive Beighton score of 7/9), velvety skin with visible scars, and a systolic heart murmur. Echocardiography reveals mitral valve prolapse. Skin biopsy shows abnormal collagen fibril organization. Which of the following collagen types is primarily defective in this patient's condition?

Explanation

## Clinical Diagnosis: Ehlers-Danlos Syndrome (EDS) ### Key Clinical Features **Key Point:** This patient presents with the classic triad of EDS: joint hypermobility, skin hyperextensibility, and tissue fragility (easy bruising). The presence of mitral valve prolapse and abnormal collagen fibrils on biopsy points to a specific EDS subtype. ### Collagen Pathology in EDS | EDS Type | Defective Collagen | Molecular Defect | Key Features | |---|---|---|---| | **Classical (Type I/II)** | Type I & III | Collagen synthesis or cross-linking | Skin hyperextensibility, atrophic scars, joint hypermobility | | **Vascular (Type IV)** | Type III | COL3A1 mutations | Arterial rupture, GI perforation, thin skin | | **Hypermobility (Type III)** | Type III | Tenascin-X or FKBP14 defects | Joint hypermobility, minimal skin changes, MVP | | **Kyphoscoliotic (Type VI)** | Type I | Lysyl hydroxylase deficiency | Severe kyphoscoliosis, eye fragility | **High-Yield:** Type III collagen (encoded by COL3A1) is the primary structural component of skin, blood vessels, and connective tissue. In EDS, abnormal Type III collagen results in: - Defective fibril formation and organization - Loss of tensile strength in skin and vessels - Joint instability due to weakened ligaments - Mitral valve prolapse from myxomatous degeneration ### Why Type III Collagen? 1. **Skin manifestations**: Type III collagen comprises ~80% of dermal collagen; defects cause hyperextensibility and abnormal scar formation. 2. **Joint hypermobility**: Type III collagen stabilizes ligaments and joint capsules; its deficiency allows excessive movement. 3. **Cardiovascular involvement**: Type III collagen is critical for vascular wall integrity and valve structure; mutations predispose to MVP and arterial fragility. 4. **Biopsy findings**: Electron microscopy shows irregular, thin collagen fibrils—characteristic of Type III collagen defects. **Clinical Pearl:** The Beighton score ≥7 in an adult with skin hyperextensibility and MVP is virtually diagnostic of hypermobility-type EDS, which is almost always due to Type III collagen or tenascin-X defects. ### Mechanism of Collagen Defect ```mermaid flowchart TD A[COL3A1 Gene Mutation]:::outcome --> B[Abnormal Type III Collagen Synthesis]:::outcome B --> C{Structural Consequences}:::decision C -->|Skin| D[Defective Fibril Organization]:::action C -->|Vessels| E[Weakened Arterial Walls]:::action C -->|Ligaments| F[Joint Hypermobility]:::action C -->|Valve| G[Myxomatous Degeneration MVP]:::action D --> H[Hyperextensibility + Easy Bruising]:::outcome E --> I[Risk of Rupture]:::urgent F --> J[Positive Beighton Score]:::outcome G --> K[Mitral Valve Prolapse]:::outcome ``` **Mnemonic:** **EDS-HT = Type III** (Hypermobility-type EDS = Type III collagen defect) [cite:Robbins 10e Ch 3]