A 6-year-old boy from rural India presents with bleeding gums, poor wound healing, and petechial rash. Biochemical analysis shows defective hydroxylation of proline and lysine residues in collagen. A 45-year-old woman presents with joint stiffness, skin thickening, and progressive fibrosis of internal organs. Collagen cross-linking is abnormally increased. Which biochemical feature best discriminates Scurvy from Systemic Sclerosis?

Explanation

## Scurvy vs. Systemic Sclerosis: Biochemical Discrimination ### Pathophysiologic Basis **Key Point:** Scurvy results from vitamin C deficiency, which impairs the hydroxylation of proline and lysine residues in collagen. This step is catalyzed by prolyl hydroxylase and lysyl hydroxylase, both requiring vitamin C as a cofactor. Systemic Sclerosis (SSc) is an autoimmune fibrotic disorder characterized by excessive collagen synthesis and abnormal cross-linking, but hydroxylation is normal. ### Biochemical Comparison Table | Biochemical Feature | Scurvy | Systemic Sclerosis | |-------------------|--------|---------------------| | **Proline/Lysine hydroxylation** | **Defective** (↓ due to ↓ Vit C) | Normal | | **Collagen synthesis** | Normal/↓ | **Increased** (↑↑) | | **Collagen cross-linking** | Defective (↓ lysyl oxidase activity) | **Abnormally increased** | | **Collagen stability** | Unstable, easily degraded | Excessive, pathologic accumulation | | **Underlying cause** | Nutritional (Vit C deficiency) | Autoimmune (TGF-β, fibroblast activation) | | **Reversibility** | Yes (with Vit C replacement) | Progressive, irreversible | ### Clinical-Biochemical Correlation **High-Yield:** The defective hydroxylation in scurvy leads to: 1. **Unstable collagen** — cannot form proper triple helix 2. **Poor wound healing** — collagen is rapidly degraded 3. **Bleeding gums** — fragile blood vessel walls 4. **Petechiae and ecchymoses** — due to vascular fragility In contrast, systemic sclerosis has: 1. **Excessive collagen deposition** — fibrotic skin, organs 2. **Abnormal cross-linking** — collagen becomes rigid and inelastic 3. **Progressive fibrosis** — affects skin, lungs, heart, kidneys **Clinical Pearl:** A scurvy patient with a bleeding gum will show defective collagen under electron microscopy (irregular fibrils, poor cross-links). A systemic sclerosis patient will show excessive, tightly cross-linked collagen bundles. **Mnemonic:** **Scurvy = Synthesis defect (hydroxylation)** → unstable collagen; **Sclerosis = Synthesis excess** → excessive collagen. ```mermaid flowchart TD A[Collagen Disorder]:::outcome --> B{Biochemical defect?}:::decision B -->|Defective hydroxylation of Pro/Lys| C[Scurvy]:::outcome B -->|Excessive synthesis + abnormal cross-linking| D[Systemic Sclerosis]:::outcome C --> E[Vitamin C deficiency]:::action D --> F[Autoimmune fibroblast activation]:::action E --> G[Reversible with Vit C]:::action F --> H[Progressive, irreversible]:::urgent ``` [cite:Robbins 10e Ch 5; KD Tripathi 8e Ch 12]