USMLE Step 1 & 2 Inflammation and Repair
Last updated: May 2, 2026
Inflammation and Repair questions are one of the highest-leverage areas to study for the USMLE Step 1 & 2. This guide breaks down the rule, the elements you need to recognize, the named traps that catch most students, and a memory aid that scales to test day. Read it once, then practice the same sub-topic adaptively in the app.
The rule
Acute inflammation is a rapid, neutrophil-dominant vascular and cellular response to injury (minutes to days), driven by histamine, prostaglandins, leukotrienes, complement, and cytokines (TNF, IL-1, IL-6). Chronic inflammation is a slower, mononuclear-cell response (macrophages, lymphocytes, plasma cells) that runs for weeks to years and is the substrate for granuloma formation, fibrosis, and tissue destruction. Repair proceeds through three overlapping phases — hemostasis/inflammation (0–3 days), proliferation with granulation tissue (3–14 days), and remodeling (weeks to years) — and the outcome depends on whether the parenchymal cells are labile, stable, or permanent and whether the stromal scaffold is intact.
Elements breakdown
Vascular phase of acute inflammation
Immediate hemodynamic and permeability changes producing the cardinal signs.
- transient vasoconstriction then vasodilation
- increased vascular permeability via histamine
- endothelial gap formation in postcapillary venules
- exudation of protein-rich fluid
- stasis and margination of leukocytes
Common examples:
- bee sting wheal-and-flare
- early sunburn erythema
Cellular phase — neutrophil recruitment
Sequential adhesion and emigration of neutrophils to the injury site.
- margination along venule walls
- rolling via selectins (E-, P-, L-selectin)
- tight adhesion via integrins binding ICAM-1/VCAM-1
- transmigration (diapedesis) through PECAM-1
- chemotaxis along C5a, LTB4, IL-8, bacterial peptides
Common examples:
- leukocyte adhesion deficiency type 1 (CD18 defect)
- leukocyte adhesion deficiency type 2 (sialyl-Lewis X defect)
Chemical mediators
Soluble signals that drive vascular changes, leukocyte recruitment, and pain/fever.
- histamine — vasodilation, permeability
- prostaglandins (PGE2) — pain, fever, vasodilation
- leukotrienes (LTB4 chemotactic; LTC4/D4/E4 bronchospasm)
- complement (C3a, C5a anaphylatoxins; MAC)
- cytokines TNF, IL-1, IL-6 — fever, acute-phase response
- bradykinin — pain, permeability
Common examples:
- NSAIDs block COX → no PGs
- zileuton blocks 5-LOX
Chronic inflammation patterns
Persistent inflammation with mononuclear cells and tissue restructuring.
- macrophage and lymphocyte infiltrate
- plasma cells producing antibodies
- granuloma formation (epithelioid macrophages, multinucleated giant cells)
- fibroblast proliferation and collagen deposition
- tissue destruction concurrent with repair
Common examples:
- TB caseating granulomas
- sarcoidosis non-caseating granulomas
- RA pannus
- H. pylori gastritis
Granuloma subtypes
Organized collections of activated macrophages with characteristic features.
- caseating — central necrosis, infectious etiology
- non-caseating — no central necrosis, often autoimmune
- Th1-driven, IFN-γ and TNF dependent
- epithelioid macrophages and Langhans giant cells
- requires CD4 T-cell help
Common examples:
- TB, fungal infections (caseating)
- sarcoidosis, Crohn, foreign body (non-caseating)
Phases of wound healing
Time-ordered repair following tissue injury.
- hemostasis 0–hours: platelet plug, fibrin
- inflammation hours–3d: neutrophils then macrophages
- proliferation 3–14d: granulation tissue, angiogenesis, re-epithelialization
- remodeling 1wk–years: type III → type I collagen via MMPs
- wound strength plateaus at ~80% of original
Common examples:
- surgical wound timeline
- myocardial infarct scar maturation
Cell types by regenerative capacity
Determines whether tissue regenerates or scars.
- labile — continuously dividing (skin, gut, marrow)
- stable — quiescent, can re-enter cycle (hepatocytes, renal tubules)
- permanent — non-dividing (neurons, cardiac myocytes, skeletal muscle)
- intact stroma → regeneration
- destroyed stroma → fibrosis
Common examples:
- hepatocyte regeneration after partial hepatectomy
- MI heals as fibrous scar
Pathologic repair
Repair gone wrong — excessive, deficient, or contracted.
- hypertrophic scar — collagen within wound borders
- keloid — type III collagen extending beyond borders
- wound dehiscence — mechanical failure
- contracture — burns, palmar fascia
- chronic non-healing ulcer — diabetes, venous stasis, pressure
Common examples:
- earlobe keloid after piercing
- Dupuytren contracture
- diabetic foot ulcer
Common patterns and traps
The Granuloma Differential Split
Step 1 histology questions that show epithelioid macrophages and giant cells force you to split caseating vs non-caseating, then map the pattern to a specific etiology. Caseating with acid-fast bacilli on Ziehl-Neelsen = TB; caseating with broad-based budding yeast = Blastomyces; non-caseating with bilateral hilar lymphadenopathy and hypercalcemia = sarcoid; non-caseating with transmural skip lesions in terminal ileum = Crohn. The trap is picking the right granuloma type but the wrong organism, or vice versa.
A choice naming a non-granulomatous disease (e.g., 'rheumatoid arthritis') when the histology clearly shows organized epithelioid macrophages, or naming sarcoidosis when caseation is described.
The LAD Selectin-vs-Integrin Trap
Leukocyte adhesion deficiency questions hinge on whether the defect blocks rolling or firm adhesion. LAD-1 = β2-integrin (CD18) deficiency → no firm adhesion → neutrophilia, delayed umbilical cord separation, recurrent skin/mucosal infections without pus. LAD-2 = sialyl-Lewis X defect → no rolling → similar phenotype plus growth and developmental delay. Candidates conflate the two or pick complement deficiency.
A delayed-cord-separation vignette where the wrong choice names a complement component or a phagocyte oxidative-burst enzyme rather than the integrin/selectin step that actually fits.
The NSAID Mechanism Decoy
Pharmacology overlap: NSAIDs inhibit COX-1/COX-2 → blocks prostaglandins (vasodilation, pain, fever) but NOT leukotrienes. Aspirin is irreversible (acetylates COX); other NSAIDs are reversible. The wrong-answer pattern offers leukotriene inhibition or lipoxygenase inhibition for an aspirin question, exploiting the candidate's vague memory that NSAIDs 'block inflammation.'
A choice claiming aspirin works by blocking 5-lipoxygenase or by neutralizing histamine release, when the correct answer is irreversible cyclooxygenase inhibition.
The Cell-Type Regeneration Rule
Stems describing post-injury healing test whether you know labile vs stable vs permanent cells. Skin and gut regenerate completely from stem cells in basal layer/crypts; hepatocytes can regenerate after partial hepatectomy if the reticulin scaffold survives; cardiac myocytes and neurons cannot — they always heal by scar (gliosis in CNS). The trap pairs a permanent-cell tissue with a regeneration choice or a labile-cell tissue with a fibrous-scar choice.
A post-MI vignette where the wrong choice describes 'cardiac myocyte regeneration via resident stem cells' instead of granulation tissue and collagen scar.
The Wound Timeline Misattribution
Time-from-injury stems demand precise mapping: 0–24h neutrophils, 1–3d macrophages dominate, 3–7d granulation tissue with angiogenesis, 1–2wk re-epithelialization complete, weeks-to-months remodeling with type III → type I collagen via matrix metalloproteinases. Wrong answers shift the cellular infiltrate by one phase or assign collagen remodeling to the inflammatory phase.
A choice describing 'predominantly neutrophilic infiltrate' for a wound at day 7, or 'type I collagen deposition' as the dominant feature at day 3.
How it works
Picture Mr. Alvarez stepping on a rusty nail. Within minutes mast cells degranulate histamine, postcapillary venules become leaky, and the foot turns red, hot, and swollen — the vascular phase of acute inflammation. Over the next hours, neutrophils roll on selectins, firmly adhere via β2-integrins binding ICAM-1, and crawl out toward C5a and bacterial f-Met-Leu-Phe peptides. If he clears the bacteria and the dermal stroma is intact, macrophages (M2) drive granulation tissue, fibroblasts lay down type III collagen, capillaries sprout under VEGF, and re-epithelialization closes the wound — by week three remodeling has cross-linked the collagen into the stronger type I form, though the scar will only ever reach about 80% of original tensile strength. If instead the organism is something like Mycobacterium tuberculosis that the neutrophils cannot kill, CD4 Th1 cells release IFN-γ, macrophages transform into epithelioid cells and fuse into Langhans giant cells, and you get a caseating granuloma — the prototype chronic inflammation pattern that the USMLE will test repeatedly. Recognize the timeline (neutrophils first 24h → macrophages 2–3d → granulation 3–14d → remodeling weeks-to-years) and the cell-type rule (permanent cells scar, labile cells regenerate) and most stem will resolve to a single answer.
Worked examples
A defect in which of the following molecules is most likely responsible for this patient's findings?
- A E-selectin on endothelial cells
- B β2-integrin (CD18) on neutrophils ✓ Correct
- C NADPH oxidase in phagolysosomes
- D C3 component of complement
Why B is correct: This is leukocyte adhesion deficiency type 1. Delayed umbilical cord separation (>30 days), recurrent bacterial infections without pus, and marked peripheral neutrophilia are classic. Rolling is intact (selectins work), but firm adhesion via β2-integrins (CD11/CD18 = LFA-1, Mac-1) binding endothelial ICAM-1 fails, so neutrophils cannot transmigrate.
Why each wrong choice fails:
- A: E-selectin defects (or sialyl-Lewis X ligand defects in LAD-2) impair rolling, not firm adhesion. The vignette specifically states rolling is intact, so the defect is downstream at the integrin step. (The LAD Selectin-vs-Integrin Trap)
- C: NADPH oxidase deficiency causes chronic granulomatous disease — neutrophils reach the site and form granulomas but cannot generate respiratory burst to kill catalase-positive organisms. CGD does not cause delayed cord separation or migration failure.
- D: C3 deficiency causes recurrent pyogenic infections with encapsulated organisms because of impaired opsonization and chemotaxis (loss of C3a/C5a), but it does not produce neutrophilia of 38,000 or delayed cord separation, and pus is typically still formed.
Which of the following best explains the elevated serum calcium in this patient?
- A Parathyroid hormone-related peptide secreted by lymph node tumor cells
- B Activated macrophages within granulomas expressing 1α-hydroxylase ✓ Correct
- C Lytic bone lesions from metastatic disease
- D Autonomous parathyroid adenoma
Why B is correct: This is sarcoidosis: bilateral hilar lymphadenopathy, erythema nodosum, non-caseating granulomas, elevated ACE. The hypercalcemia is mediated by epithelioid macrophages within the granulomas, which constitutively express 1α-hydroxylase and convert 25-OH-D to 1,25-(OH)₂-D outside normal feedback control, increasing intestinal calcium absorption.
Why each wrong choice fails:
- A: PTHrP-mediated humoral hypercalcemia of malignancy is classically seen in squamous cell carcinoma of the lung, renal cell carcinoma, and breast cancer — not in benign granulomatous disease, and the biopsy shows non-caseating granulomas, not malignant cells. (The Granuloma Differential Split)
- C: There is no imaging or clinical evidence of skeletal metastases, the biopsy shows granulomas (not malignancy), and lytic-lesion hypercalcemia would typically present with bone pain and an abnormal skeletal survey, none of which are described.
- D: Primary hyperparathyroidism is an important hypercalcemia cause, but the clinical picture (hilar adenopathy, erythema nodosum, non-caseating granulomas, elevated ACE, and elevated 1,25-D rather than PTH) is diagnostic of sarcoidosis, not parathyroid adenoma.
Which of the following best explains the pathophysiology of this complication?
- A Wound has reached only ~10% of original tensile strength because remodeling has not yet begun ✓ Correct
- B Excessive type III collagen deposition extending beyond wound margins
- C Premature transition from inflammatory phase to remodeling phase
- D Failure of re-epithelialization due to chronic neutrophilic infiltrate
Why A is correct: This is wound dehiscence. At one week post-op, a wound is still in early proliferation — collagen is being laid down but cross-linking and the type III → type I conversion of remodeling have barely started, so tensile strength is only about 10% of normal. Diabetes, smoking, and Valsalva (coughing) further impair healing and raise intra-abdominal pressure, mechanically overwhelming the immature scar.
Why each wrong choice fails:
- B: That describes a keloid, where type III collagen extends beyond the original wound borders. Keloids are a cosmetic problem of excessive repair, not a cause of acute fascial dehiscence on day 7. (The Wound Timeline Misattribution)
- C: The problem is that remodeling has not yet started — it begins after several weeks. Premature remodeling is not a recognized mechanism of dehiscence; the issue is biomechanical inadequacy of an immature wound. (The Wound Timeline Misattribution)
- D: Failed re-epithelialization produces a chronic non-healing surface ulcer, not sudden fascial separation with bowel evisceration. The fascia, not the epidermis, is the load-bearing layer that has failed here.
Memory aid
Cardinal signs — 'rubor, calor, dolor, tumor, functio laesa' (red, hot, painful, swollen, loss of function). Healing timeline — 'NMG-RR' (Neutrophils, Macrophages, Granulation, Remodeling). Granuloma triggers — 'Mighty Strong Bacteria Find Cats' (Mycobacteria, Sarcoid, Berylliosis, Fungi, Cat-scratch).
Key distinction
Caseating vs non-caseating granuloma — caseating means central necrosis (cheese-like) and points to mycobacterial or fungal infection; non-caseating points to sarcoidosis, Crohn disease, foreign body, or berylliosis. The histology question almost always hinges on whether you see central pink amorphous debris.
Summary
Acute inflammation = neutrophils + vascular leak in days; chronic inflammation = mononuclear cells + granulomas/fibrosis in weeks; repair outcome depends on cell-type regenerative capacity and whether the stromal scaffold survived.
Practice inflammation and repair adaptively
Reading the rule is the start. Working USMLE Step 1 & 2-format questions on this sub-topic with adaptive selection, watching your mastery score climb in real time, and seeing the items you missed return on a spaced-repetition schedule — that's where score lift actually happens. Free for seven days. No credit card required.
Start your free 7-day trialFrequently asked questions
What is inflammation and repair on the USMLE Step 1 & 2?
Acute inflammation is a rapid, neutrophil-dominant vascular and cellular response to injury (minutes to days), driven by histamine, prostaglandins, leukotrienes, complement, and cytokines (TNF, IL-1, IL-6). Chronic inflammation is a slower, mononuclear-cell response (macrophages, lymphocytes, plasma cells) that runs for weeks to years and is the substrate for granuloma formation, fibrosis, and tissue destruction. Repair proceeds through three overlapping phases — hemostasis/inflammation (0–3 days), proliferation with granulation tissue (3–14 days), and remodeling (weeks to years) — and the outcome depends on whether the parenchymal cells are labile, stable, or permanent and whether the stromal scaffold is intact.
How do I practice inflammation and repair questions?
The fastest way to improve on inflammation and repair is targeted, adaptive practice — working questions that focus on your specific weak spots within this sub-topic, getting immediate feedback, and revisiting items you missed on a spaced-repetition schedule. Neureto's adaptive engine does this automatically across the USMLE Step 1 & 2; start a free 7-day trial to see your sub-topic mastery climb in real time.
What's the most important distinction to remember for inflammation and repair?
Caseating vs non-caseating granuloma — caseating means central necrosis (cheese-like) and points to mycobacterial or fungal infection; non-caseating points to sarcoidosis, Crohn disease, foreign body, or berylliosis. The histology question almost always hinges on whether you see central pink amorphous debris.
Is there a memory aid for inflammation and repair questions?
Cardinal signs — 'rubor, calor, dolor, tumor, functio laesa' (red, hot, painful, swollen, loss of function). Healing timeline — 'NMG-RR' (Neutrophils, Macrophages, Granulation, Remodeling). Granuloma triggers — 'Mighty Strong Bacteria Find Cats' (Mycobacteria, Sarcoid, Berylliosis, Fungi, Cat-scratch).
What's a common trap on inflammation and repair questions?
Confusing caseating with non-caseating granulomas
What's a common trap on inflammation and repair questions?
Mixing up selectin vs integrin defects in LAD
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