Which feature best distinguishes an entrance wound from an exit wound in a firearm injury?

Explanation

## Entrance vs Exit Wounds: Key Discriminating Features ### Abrasion Collar (Contusion Collar) **Key Point:** The abrasion collar is the most reliable and consistent feature distinguishing entrance from exit wounds. It is a thin rim of abraded (scraped) epidermis around the entrance wound caused by the projectile's friction as it penetrates the skin. **High-Yield:** Abrasion collars are: - Present in ~95% of entrance wounds - Absent in exit wounds (no outward scraping) - Visible even in contact or close-range shots - Independent of clothing or intermediate targets ### Bone Bevelling Pattern | Feature | Entrance Wound | Exit Wound | |---------|---|---| | **Bone bevelling** | Inward (inverted) bevelling — inner table is smaller | Outward bevelling — outer table is smaller | | **Mechanism** | Projectile pushes bone inward, creating cone-shaped defect | Projectile exits forcefully, pushing bone outward | | **Reliability** | Highly reliable when skull is involved | Highly reliable when skull is involved | **Clinical Pearl:** In soft tissue only (chest, abdomen, limbs), bone bevelling is absent, making the abrasion collar the primary discriminator. ### Secondary Features (Less Reliable) - **Soot & powder tattooing:** Present only at entrance wounds in close-range shots; absent if distance is >1 m or if intermediate objects block deposition - **Wound margins:** Entrance wounds typically have inverted edges; exit wounds have everted edges — but this varies with tissue elasticity and projectile deformation - **Diameter:** Exit wounds are often larger, but a deformed projectile at entrance can create a large wound **Mnemonic:** **ABIDE** — Abrasion collar, Bevelling (inward), Inverted edges, Diameter (variable), Entry point ### Why Abrasion Collar Wins It is: 1. Present in nearly all entrance wounds 2. Absent in all exit wounds 3. Independent of range, ammunition type, or intermediate objects 4. Visible microscopically even if not grossly apparent [cite:Reddy Forensic Medicine 33e Ch 8]