A 3-week-old boy is brought to the pediatrician by his parents for a circumcision. The circumcision was uncomplicated; however, after a few hours, the diaper contained blood, and the bleeding has not subsided. A complete blood count was ordered, which was significant for a platelet count of 70,000/mm3. On peripheral blood smear, the following was noted (figure A). The prothrombin time was 12 seconds, partial thromboplastin time was 32 seconds, and bleeding time was 13 minutes. On platelet aggregation studies, there was no response with ristocetin. This result was not corrected with the addition of normal plasma. There was a normal aggregation response with the addition of ADP. Which of the following is most likely true of this patient's underlying disease?

Adding epinephrine would not lead to platelet aggregation

An investigator is studying the structural integrity of collagen. Human fibroblasts are cultured on a medium and different enzymes are applied. One of the cultures is supplemented with an enzyme that inhibits lysyl oxidase, preventing the formation of covalent cross-links between collagen α-chains. Which of the following processes is most likely to be impaired as a result?

Internal elastic lamina formation

Cartilaginous growth plate mineralization

A 16-year-old male presents to the emergency department with a hematoma after falling during gym class. He claims that he has a history of prolonged nosebleeds and bruising/bleeding after minor injuries. Physical exam is unrevealing other than the hematoma. Labs are obtained showing an increased bleeding time and an abnormal ristocetin cofactor assay. Coagulation assays reveal an increased partial thromboplastin time (PTT) but a normal prothrombin time (PT). The patient is given desmopressin and is asked to avoid aspirin. Which of the following findings is most likely directly involved in the etiology of this patient's presentation?

Decreased levels or dysfunction of von Willebrand factor

A 45-year-old man comes to his primary care provider for a routine visit. The patient mentions that while he was cooking 5 days ago, he accidentally cut himself with a meat cleaver and lost the skin at the tip of his finger. After applying pressure and ice, the bleeding stopped and he did not seek treatment. The patient is otherwise healthy and does not take any daily medications. The patient’s temperature is 98.2°F (36.8°C), blood pressure is 114/72 mmHg, pulse is 60/min, and respirations are 12/min. On exam, the patient demonstrates a 0.5 x 0.3 cm wound on the tip of his left third finger. No bone is involved, and the wound is red, soft, and painless. There are no signs of infection. Which of the following can be expected on histopathological examination of the wounded area?

Deposition of type III collagen

Epithelial cell migration from the wound borders

Neutrophil migration into the wound

A 38-year-old, working, first-time mother brings her 9-month-old male infant to the pediatrician for "wounds that simply won't heal" and bleeding gums. She exclaims, "I have been extra careful with him making sure to not let him get dirty, I boil his baby formula for 15 minutes each morning before I leave for work to give to the caregiver, and he has gotten all of his vaccinations." This infant is deficient in a molecule that is also an essential co-factor for which of the following reactions?

Conversion of dopamine to norepinephrine

Conversion of homocysteine to methionine

Conversion of alpha ketoglutarate to succinyl-CoA

Conversion of pyruvate to acetyl-CoA

Conversion of pyruvate to oxaloacetate

A previously healthy 5-year-old boy is brought to the emergency department 15 minutes after sustaining an injury to his right hand. His mother says that she was cleaning the bathroom when he accidentally knocked over the drain cleaner bottle and spilled the liquid onto his hand. On arrival, he is crying and holding his right hand in a flexed position. His temperature is 37.7°C (99.8°F), pulse is 105/min, respirations are 25/min, and blood pressure is 105/65 mm Hg. Examination of the right hand shows a 4 x 4 cm area of reddened, blistered skin. The area is very tender to light touch. His ability to flex and extend the right hand are diminished. Radial pulses are palpable. Capillary refill time is less than 3 seconds. Which of the following is the most appropriate next step in management?

Apply split-thickness skin graft

Phases of wound healing — USMLE Lesson

Hemostasis - The First Plug

Initiation: Vessel injury exposes subendothelial collagen and von Willebrand Factor (vWF).
Step 1: Vasoconstriction
- A transient neurogenic reflex, reinforced by endothelin.
- Immediately reduces blood flow.
Step 2: Primary Hemostasis (Platelet Plug)
- Adhesion: Platelet receptor GpIb binds to vWF on exposed collagen.
- Activation: Platelets change shape and degranulate, releasing ADP and Thromboxane A₂ ($TXA_2$).
- Aggregation: Fibrinogen cross-links platelets via the GpIIb/IIIa receptor, forming the initial plug.

Primary Hemostasis: Platelet Adhesion and Aggregation

⭐ Glanzmann Thrombasthenia is an autosomal recessive disorder caused by a deficiency of GpIIb/IIIa, leading to impaired platelet aggregation and a severe bleeding diathesis.

Inflammation - The Clean‑Up Crew

Timeline: Day 0-3, initiated by the coagulation cascade and complement activation.
Vascular Response: Transient vasoconstriction (hemostasis) is followed by vasodilation and increased permeability.
Cellular Influx:
- Neutrophils (PMNs): Peak at 24-48 hours. They are the primary defense against infection, performing phagocytosis of bacteria and debris.
- Macrophages (Monocytes): Become dominant by 48-72 hours. They are essential for debridement and orchestrate the transition to proliferation by releasing growth factors (PDGF, TGF-β, VEGF).
- 📌 Mnemonic: Neutrophils Navigate, Macrophages Manage.

Four phases of wound healing: hemostasis to remodeling

⭐ Macrophages are the critical directors of wound repair. Their depletion results in impaired debridement and a complete failure to initiate the proliferative phase, leading to a chronic wound state.

Proliferation - The Rebuilding

Timeline: Day 3 to weeks.
Hallmark: Formation of pink, soft granulation tissue.
- Fibroplasia: Fibroblasts (driven by PDGF, FGF, TGF-β) migrate and deposit ground substance and Type III collagen.
- Angiogenesis: New capillary formation (driven by VEGF, FGF) gives the tissue its granular look.
- Epithelialization: Keratinocytes migrate from wound edges to cover the surface.
Wound Contraction:
- Myofibroblasts (specialized fibroblasts with actin filaments) contract, shrinking the wound.

Histology of Granulation Tissue

⭐ The initial scaffold is made of Type III collagen. This is later replaced by stronger Type I collagen in the remodeling phase, a critical concept for understanding scar strength.

Remodeling - The Final Polish

Timeline: Begins week 3; can last for 1-2 years.
Primary Goal: ↑ Tensile strength through collagen reorganization.
- Type III collagen is replaced by stronger Type I collagen.
- Key enzymes: Matrix Metalloproteinases (MMPs) degrade collagen; Lysyl oxidase cross-links it.
Tensile Strength:
- Plateaus at ~80% of the original tissue's strength after several months.
Clinical Appearance: The scar flattens, softens, and pales over time.
Pathology: Imbalance in collagen synthesis/degradation leads to hypertrophic scars or keloids.

⭐ The ratio of Type I to Type III collagen is a key indicator of wound maturity. A mature scar approaches the normal skin ratio of approximately 4:1.

High‑Yield Points - ⚡ Biggest Takeaways

Inflammatory phase: Neutrophils arrive first, but macrophages are critical for transitioning to the proliferative phase.

Proliferative phase: Defined by granulation tissue formation, with fibroblasts depositing Type III collagen and myofibroblasts causing contraction.

Remodeling phase: Type III collagen is replaced by stronger Type I collagen, a process that can last for more than a year.

Macrophages are the most critical cell type, directing the entire repair process.

Final wound strength is at most ~80% of the original tissue.

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