Regarding the biochemistry and molecular biology of collagen in wound healing, all of the following are correct EXCEPT:

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Accepted Answer

D. Type III collagen is laid down first during wound healing and is completely replaced by Type I collagen by the end of the inflammatory phase. ## Collagen Structure, Synthesis, and Remodeling in Wound Healing ### Collagen Types and Timeline **Key Point:** Type III collagen is deposited early in wound healing but is NOT completely replaced by Type I collagen by the end of the inflammatory phase. Remodeling occurs over weeks to months. ### Collagen Synthesis and Maturation ```mermaid flowchart TD A[Fibroblast synthesizes procollagen]:::action --> B[Hydroxylation of Pro & Lys
requires Vitamin C]:::action B --> C[Triple helix formation]:::outcome C --> D[Secretion of Type III & Type I collagen]:::action D --> E[Lysyl oxidase cross-linking
Lys → Aldehyde]:::action E --> F[Collagen maturation & remodeling
weeks to months]:::action F --> G[Type III → Type I ratio decreases
Tensile strength increases]:::outcome ``` ### Analysis of Each Option | Statement | Accuracy | Explanation | |-----------|----------|-------------| | Type I collagen predominates in mature scars | ✓ Correct | Type I is ~80% of mature scar; provides tensile strength | | Lysyl oxidase oxidizes Lys/Hyl residues | ✓ Correct | Forms aldehyde cross-links (allysine); requires Cu²⁺ cofactor | | Vitamin C hydroxylates Pro/Lys | ✓ Correct | Ascorbic acid is essential; deficiency → scurvy, poor healing | | Type III completely replaced by day 5 | ✗ **WRONG** | Type III is laid down early but remodeling continues for weeks–months | ### Why Option 3 is Wrong **High-Yield:** Type III collagen is deposited during the proliferative phase (days 5–21) but is NOT completely replaced by Type I collagen by the end of the inflammatory phase (day 5). In fact: 1. **Early wound (days 5–14):** Type III:Type I ratio is ~3:1 2. **Intermediate wound (weeks 2–4):** Type III:Type I ratio is ~1:1 3. **Mature scar (>3 months):** Type III:Type I ratio is ~1:10 **Clinical Pearl:** The gradual replacement of Type III with Type I collagen is a hallmark of the remodeling phase (weeks to years). This process is driven by matrix metalloproteinases (MMPs) and tissue inhibitors of metalloproteinases (TIMPs). Premature scar revision (<3 months) risks disrupting this remodeling and should be avoided unless necessary. ### Verification of Correct Options **Option 0:** Type I collagen (80% of mature scar) provides tensile strength. Type III is more compliant but weaker. **Option 1:** Lysyl oxidase (LOX), a copper-dependent enzyme, catalyzes the oxidation of lysine and hydroxylysine residues to aldehydes (allysine, hydroxyallysine), which then condense to form Schiff bases and aldol cross-links. This is essential for collagen mechanical strength. **Option 2:** Vitamin C (ascorbic acid) is a cofactor for prolyl and lysyl hydroxylase. Without it, proline and lysine residues are not hydroxylated, preventing triple helix formation and leading to defective collagen (scurvy). **Mnemonic: VLC** — Vitamin C, Lysyl oxidase, Collagen cross-linking (the three critical biochemical steps for collagen maturation).

Answer

A. Lysyl oxidase catalyzes the oxidation of lysine and hydroxylysine residues to form aldehyde cross-links between collagen molecules

Answer

B. Type I collagen is the predominant collagen in mature scars and provides tensile strength

Answer

C. Vitamin C is essential for the hydroxylation of proline and lysine residues, which is required for collagen triple helix stability

Regarding the biochemistry and molecular biology of collagen in wound healing, all of the following are correct EXCEPT:

Explanation

Collagen Structure, Synthesis, and Remodeling in Wound Healing

Collagen Types and Timeline

Collagen Synthesis and Maturation

Analysis of Each Option

Why Option 3 is Wrong

Verification of Correct Options

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Statement	Accuracy	Explanation
Type I collagen predominates in mature scars	✓ Correct	Type I is ~80% of mature scar; provides tensile strength
Lysyl oxidase oxidizes Lys/Hyl residues	✓ Correct	Forms aldehyde cross-links (allysine); requires Cu²⁺ cofactor
Vitamin C hydroxylates Pro/Lys	✓ Correct	Ascorbic acid is essential; deficiency → scurvy, poor healing
Type III completely replaced by day 5	✗ WRONG	Type III is laid down early but remodeling continues for weeks–months