Transplant questions love to test whether you can separate host-vs-graft from graft-vs-host, and whether you know when each one happens and why. The sneaky part: the vignette usually hands you just enough clues to pick the right diagnosis—but the answer choices are engineered to punish shallow pattern recognition. Let’s do a Q-bank–style breakdown where every distractor teaches something you’ll reuse on test day.
Tag: Immunology > Transplant & Autoimmune
The Vignette (Classic USMLE Style)
A 42-year-old man with acute myelogenous leukemia undergoes an allogeneic hematopoietic stem cell transplant from an HLA-matched sibling. Three weeks later, he develops fever, a diffuse maculopapular rash (including palms/soles), watery diarrhea, abdominal cramping, and elevated liver enzymes. Exam shows erythematous, pruritic rash. Labs show rising bilirubin and alkaline phosphatase.
Question: What is the most likely diagnosis/mechanism?
The Correct Answer: Acute Graft-vs-Host Disease (GVHD)
Why it fits
This is the Step-classic triad after allogeneic bone marrow transplant:
- Skin: maculopapular rash (can become desquamating)
- GI: profuse watery diarrhea, abdominal pain
- Liver: cholestatic picture (↑ bilirubin, ↑ ALP, ↑ AST/ALT)
Mechanism (the “who attacks whom” rule)
- Donor immunocompetent T cells in the graft recognize recipient tissues as foreign.
- This causes a type IV hypersensitivity–like T-cell–mediated injury.
Key sentence to memorize:
GVHD = graft’s T cells attack the host (recipient).
Timing
- Acute GVHD: typically days to weeks post-transplant (often within the first ~100 days)
- Chronic GVHD: later, can resemble autoimmune disease (e.g., scleroderma-like skin changes, dry eyes/mouth)
High-yield risk factors
- Allogeneic HSCT (bone marrow/stem cell transplant) is the big one.
- Also possible after liver transplant (less common) because the liver contains many immune cells.
- Increased risk with HLA mismatch (but can occur even when matched due to minor antigens).
Prevention & treatment (Step-ready)
- Prevention: T-cell depletion of graft + immunosuppression (e.g., methotrexate, cyclosporine/tacrolimus)
- Treatment: high-dose glucocorticoids are often first-line; additional immunosuppression depending on severity.
Why Every Other Answer Choice Is Wrong (and What It Really Describes)
Below is a “distractor decoder.” Train yourself to do this mentally—even when you already know the right answer.
Distractor 1: Hyperacute transplant rejection (preformed anti-ABO or anti-HLA antibodies)
Why it’s tempting: “Transplant” + “bad outcome” can make people click rejection automatically.
Why it’s wrong here:
- Hyperacute rejection occurs minutes to hours after transplant, not 3 weeks.
- It’s primarily about solid organ grafts (kidney, heart), not HSCT.
- Caused by preformed recipient IgG against donor antigens → complement activation → thrombosis and necrosis.
Buzzwords to expect instead:
- Immediate graft failure, cyanosis, mottling
- Widespread thrombosis, fibrinoid necrosis
High-yield line:
Hyperacute = preformed antibodies = minutes-hours.
Distractor 2: Acute cellular rejection of a solid organ (recipient T cells attack graft)
Why it’s tempting: “Acute” and “T cells” sounds close to GVHD.
Why it’s wrong here:
- Acute cellular rejection is host-vs-graft, not graft-vs-host.
- Typically occurs weeks to months after solid organ transplant (kidney, heart, liver).
- Would present with organ-specific dysfunction (e.g., rising creatinine in kidney transplant), not diffuse rash/diarrhea.
Mechanism:
- Recipient CD8+ cytotoxic T cells directly attack donor cells
- Recipient CD4+ T cells drive inflammation via cytokines
High-yield line:
Acute cellular rejection = recipient T cells attack graft tissue (host-vs-graft).
Distractor 3: Chronic rejection (vascular smooth muscle proliferation, fibrosis)
Why it’s tempting: “Later complications of transplant” often get lumped together.
Why it’s wrong here:
- Chronic rejection develops over months to years, not weeks.
- Presents as progressive graft failure with ischemic changes.
- Pathology: intimal thickening, fibrosis, and luminal narrowing (think “graft vasculopathy”).
Buzzwords:
- “progressive” decline in graft function
- “arteriolosclerosis” / “obliterative intimal fibrosis”
High-yield line:
Chronic rejection = fibrosis + vascular narrowing = months-years.
Distractor 4: Post-transplant lymphoproliferative disorder (PTLD) due to EBV
Why it’s tempting: Immunosuppression + transplant = lymphoid malignancy risk.
Why it’s wrong here:
- PTLD typically presents with:
- fever, weight loss, lymphadenopathy
- extranodal masses
- EBV-driven B-cell proliferation
- Not the classic skin + GI + liver triad, and not usually 3-week onset with profuse diarrhea as the core feature.
High-yield line:
PTLD = EBV-driven B-cell proliferation in immunosuppressed transplant patients.
Distractor 5: Serum sickness (Type III hypersensitivity) from antithymocyte globulin or other biologics
Why it’s tempting: Rash + systemic symptoms + immune phenomenon.
Why it’s wrong here:
- Serum sickness classically causes:
- fever
- urticarial rash
- arthralgias
- lymphadenopathy
- proteinuria (immune complex deposition)
- The vignette’s watery diarrhea + cholestatic liver injury after HSCT points much more strongly to GVHD.
High-yield line:
Serum sickness = Type III immune complex disease: fever + urticaria + arthralgia + proteinuria.
Distractor 6: Toxicity from calcineurin inhibitors (cyclosporine/tacrolimus)
Why it’s tempting: Transplant meds cause real complications.
Why it’s wrong here:
- Calcineurin inhibitor toxicity is dominated by:
- nephrotoxicity
- hypertension
- neurotoxicity (tremor, seizures)
- hyperglycemia (esp tacrolimus)
- Not the classic GVHD organ pattern.
High-yield line:
Calcineurin inhibitors → kidney + neuro issues, not rash/diarrhea/liver triad.
The “Two Rejections + One GVHD” Framework (Do This Under Time Pressure)
Quick comparison table
| Entity | Who attacks whom? | Typical setting | Timing | Hallmark findings |
|---|---|---|---|---|
| GVHD | Donor T cells → recipient | Allogeneic HSCT (± liver) | Days–weeks (acute) | Rash + diarrhea + liver dysfunction |
| Acute cellular rejection | Recipient T cells → graft | Solid organ | Weeks–months | Organ dysfunction; biopsy: lymphocytes |
| Hyperacute rejection | Recipient preformed IgG → graft | Solid organ | Minutes–hours | Thrombosis, necrosis, immediate failure |
| Chronic rejection | Mixed immune injury → graft vasculature | Solid organ | Months–years | Fibrosis, vascular narrowing, ischemia |
High-Yield Path/Immunology Nuggets USMLE Loves
GVHD is more likely when:
- The graft contains immunocompetent T cells (HSCT is the prototype)
- The recipient is immunocompromised (can’t reject donor T cells effectively)
- There’s HLA mismatch (but minor antigen differences can still trigger GVHD)
Acute vs chronic GVHD (testable pattern)
- Acute: rash, diarrhea, liver injury (often cholestatic)
- Chronic: autoimmune-like:
- scleroderma-like skin thickening
- Sjögren-like dryness
- bronchiolitis obliterans
Treatment logic (what boards want you to say)
- Prevent activation of T cells (calcineurin inhibitors)
- Suppress proliferation (methotrexate)
- Treat inflammation (glucocorticoids)
Mini “Answer Choice Autopsy” (How to Phrase It Like the Exam)
If you had to justify the correct choice in one sentence:
Allogeneic HSCT followed by rash, watery diarrhea, and cholestatic liver injury weeks later is acute GVHD caused by donor T cells attacking recipient tissues (type IV hypersensitivity).
Takeaway: The One-Liner That Saves Points
GVHD = donor T cells (from HSCT) attack host → rash + diarrhea + liver dysfunction within weeks.
If the stem says solid organ dysfunction without systemic rash/diarrhea, start thinking rejection, not GVHD.