A 21-year-old male presents to the emergency department after losing his footing and falling 20 feet off a construction scaffold. He hit his left side on a railing on the way down before landing on his left arm. He denies loss of consciousness during the event or feelings of lightheadedness. He has no significant past medical or surgical history and does not take any regular medications. Evaluation in the trauma bay revealed mild lacerations to the upper and lower extremities, pain to palpation in the distal left forearm, and bruising to the upper left quadrant of the abdomen as well as the lower left thorax. Free fluid was found in the abdomen by ultrasound, fluids were started, and he was rushed to the operating room for an exploratory laparotomy. A heavily lacerated spleen was discovered and removed. No other sources of bleeding were found. Further workup determined he suffered a non-displaced left distal radius fracture and non-displaced 9th and 10th rib fractures. Which of the following should be administered to this patient?

Total parenteral nutrition (TPN)

Open reduction internal fixation

A 48-year-old man is brought to the emergency department with a stab wound to his chest. The wound is treated in the emergency room. Three months later he develops a firm 4 x 3 cm nodular mass with intact epithelium over the site of the chest wound. On local examination, the scar is firm, non-tender, and there is no erythema. The mass is excised and microscopic examination reveals fibroblasts with plentiful collagen. Which of the following processes is most likely related to the series of events mentioned above?

Foreign body response from suturing

Poor wound healing from diabetes mellitus

A 2-year-old boy has a history of recurrent bacterial infections, especially of his skin. When he has an infection, pus does not form. His mother reports that, when he was born, his umbilical cord took 5 weeks to detach. He is ultimately diagnosed with a defect in a molecule in the pathway that results in neutrophil extravasation. Which of the following correctly pairs the defective molecule with the step of extravasation that molecule affects?

LFA-1 (integrin); tight adhesion

A 28-year-old male presents to his primary care physician with complaints of intermittent abdominal pain and alternating bouts of constipation and diarrhea. His medical chart is not significant for any past medical problems or prior surgeries. He is not prescribed any current medications. Which of the following questions would be the most useful next question in eliciting further history from this patient?

"Can you tell me more about the symptoms you have been experiencing?"

"Does the diarrhea typically precede the constipation, or vice-versa?"

"Is the diarrhea foul-smelling?"

"Please rate your abdominal pain on a scale of 1-10, with 10 being the worst pain of your life"

"Are the symptoms worse in the morning or at night?"

An immunology expert is explaining the functions of macrophages to biology students. He describes a hypothetical case scenario as follows: a potentially harmful gram-negative bacillus encounters a macrophage in the tissues. The Toll-like receptor (TLR) on the macrophage recognizes the bacterial lipopolysaccharide (LPS). The macrophage is activated by the binding of TLR with bacterial LPS and by interferon-γ (IFN-γ). Which of the following cytokines is most likely to be secreted by the activated macrophage?

Interleukin-1 receptor antagonist (IL-1RA)

Resolution of acute inflammation for USMLE Step 2 CK: High-Yield Pathology Lessons

Outcomes of Acute Inflammation - The Aftermath

Complete Resolution: Ideal outcome with restoration of normal tissue architecture. Occurs with minimal damage and high regenerative capacity.
Healing by Fibrosis (Scarring): Occurs after substantial tissue destruction or in non-regenerative tissues.
Abscess Formation: A walled-off collection of pus (neutrophils & necrotic debris), typical of pyogenic bacterial infections.
Progression to Chronic Inflammation: Results from persistent injurious stimuli or interference with normal healing.

⭐ Resolution is an active process mediated by specialized pro-resolving mediators (SPMs) like lipoxins, resolvins, and protectins.

Resolution's Key Players - The Clean-Up Crew

Macrophages (M2 Phenotype): The primary "clean-up" cells.
- Phagocytose apoptotic neutrophils & cellular debris.
- Secrete anti-inflammatory cytokines (e.g., TGF-β, IL-10).
- Produce growth factors to initiate tissue repair.
Specialized Pro-Resolving Mediators (SPMs): Actively terminate inflammation.
- Lipoxins: Generated via neutrophil-platelet interaction.
- Resolvins & Protectins: Derived from omega-3 fatty acids (EPA, DHA).
- Function: Inhibit neutrophil chemotaxis & adhesion; stimulate clearance of apoptotic cells.

Resolution of Acute Inflammation and Tissue Repair

⭐ Macrophage Polarization: The switch from pro-inflammatory M1 macrophages to anti-inflammatory M2 macrophages is a critical step for inflammation to resolve and transition into a healing phase. This switch is driven by phagocytosis of apoptotic cells and cytokines like IL-4 and IL-13.

Mechanisms of Resolution - Winding It Down

Active Termination: Resolution is an active, programmed process, not just a passive decay of the inflammatory response.
Lipid Mediator Class Switch: A pivotal shift occurs from pro-inflammatory leukotrienes to anti-inflammatory lipoxins.
- This switch is initiated by neutrophils, which then undergo apoptosis.
Specialized Pro-Resolving Mediators (SPMs):
- Includes Resolvins, Protectins, and Maresins, derived from omega-3 fatty acids.
- These molecules halt neutrophil influx and promote the clearance of apoptotic cells (efferocytosis).
Cytokine Profile Change: Shift towards anti-inflammatory cytokines, primarily IL-10 and TGF-β.

⭐ The "lipid mediator class switch" is a core principle. Aspirin-triggered lipoxins are a key pharmacologic example, enhancing resolution by mimicking this natural pathway.

Failed Resolution - The Lingering Problem

Failure of acute inflammation to resolve leads to:
- Abscess Formation: A collection of pus (neutrophils, necrotic debris) confined by a fibrous capsule. Typically caused by pyogenic organisms like S. aureus.
- Chronic Inflammation: Develops if the injurious agent persists, marked by an influx of lymphocytes and macrophages.
- Fibrosis (Scarring): Tissue replacement with collagen following substantial destruction, a process termed "organization," resulting in functional loss.

⭐ The abscess capsule is produced by fibroblasts, a key feature distinguishing it from diffuse purulent inflammation (cellulitis).

High‑Yield Points - ⚡ Biggest Takeaways

Complete resolution is the ideal outcome, where macrophages clear debris and restore normal tissue structure.

Fibrosis (scarring) results from substantial tissue destruction or in non-regenerating tissues, mediated by fibroblasts.

Abscess formation (pus) is characteristic of pyogenic bacterial infections, like Staphylococcus aureus.

Progression to chronic inflammation occurs when the injurious agent persists, involving lymphocytes and macrophages.

Key anti-inflammatory mediators driving resolution include lipoxins, resolvins, and cytokines like IL-10 and TGF-β.

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