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NBME 21 Answers

nbme21/Block 4/Question#43

A 4-year-old boy falls and lacerates his lower leg ...

Formation of fibrin clot

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  1. Hemostasis (blood clotting): Within the first few minutes of injury,
    1. platelets in the blood begin to stick to the injured site.
    2. This activates the platelets, causing a few things to happen.
    3. They change into an amorphous shape, more suitable for clotting, and they release chemical signals to promote clotting.
    4. This results in the activation of fibrin, which forms a mesh and acts as "glue" to bind platelets to each other.
    5. This makes a clot that serves to plug the break in the blood vessel, slowing/preventing further bleeding.[5][6]

  2. Inflammation: During this phase, damaged and dead cells are cleared out, along with bacteria and other pathogens or debris.
    1. This happens through the process of phagocytosis, where white blood cells "eat" debris by engulfing it. 
    2. Platelet-derived growth factors are released into the wound that cause the migration and division of cells during the proliferative phase.

  3. Proliferation (growth of new tissue): In this phase, angiogenesis, collagen deposition, granulation tissue formation, epithelialization, and wound contraction occur.
    1. In angiogenesis, vascular endothelial cells form new blood vessels.
    2. In fibroplasia and granulation tissue formation, fibroblasts grow and form a new, provisional extracellular matrix (ECM) by excreting collagen and fibronectin.
    3. Concurrently, re-epithelialization of the epidermis occurs, in which epithelial cells proliferate and 'crawl' atop the wound bed, providing cover for the new tissue.
    4. In wound contraction, myofibroblasts decrease the size of the wound by gripping the wound edges and contracting using a mechanism that resembles that in smooth muscle cells.
    5. When the cells' roles are close to complete, unneeded cells undergo apoptosis.
  4. Maturation (remodeling): During maturation and remodeling,

    1. collagen is realigned along tension lines, and cells that are no longer needed are removed by programmed cell death, or apoptosis.

  5. Approximate times of the different phases of wound healing,[10] with faded intervals marking substantial variation, depending mainly on wound size and healing conditions, but image does not include major impairments that cause chronic wounds.