You’re cruising through your immunology Q-bank and then—bam—transplant patient, rising creatinine, and a drug list that looks like a pharmacy shelf. The trick with immunosuppressants isn’t just memorizing “what blocks what.” USMLE loves forcing you to identify a drug from a side effect, a timing clue (days vs weeks vs months post-transplant), or a lab abnormality—and then not fall for distractors that are almost right.
Tag: Immunology > Transplant & Autoimmune
The Clinical Vignette (Q-bank style)
A 52-year-old man is 10 days status post deceased-donor kidney transplant. He is on prednisone, mycophenolate mofetil, and a calcineurin inhibitor. Over the past 48 hours, his urine output has decreased. Labs show:
- Serum creatinine: 2.8 mg/dL (baseline post-op 1.4 mg/dL)
- BUN: 42 mg/dL
- Potassium: 5.8 mEq/L
- Blood pressure: 164/98 mmHg
- Tacrolimus trough: elevated
Renal ultrasound shows good perfusion and no hydronephrosis. Which medication is most likely responsible for this complication?
A. Cyclosporine
B. Sirolimus
C. Mycophenolate mofetil
D. Azathioprine
E. Basiliximab
F. Infliximab
Correct Answer: A. Cyclosporine (Calcineurin inhibitor nephrotoxicity)
Even though the stem tells you he’s on “a calcineurin inhibitor,” the question asks you to pick the culprit drug from the list. Both tacrolimus and cyclosporine can do this—but only cyclosporine is offered.
Why this is the right choice
This is classic calcineurin inhibitor nephrotoxicity, which is very high yield:
- Mechanism: inhibits calcineurin → ↓ IL-2 transcription → ↓ T-cell activation
- Key adverse effect: nephrotoxicity from afferent arteriolar vasoconstriction (functional AKI)
- Associated clues:
- Hypertension
- Hyperkalemia
- Rising creatinine with no obstruction on ultrasound
- Often related to high trough levels
- Timing: can occur early post-transplant (days–weeks), especially with supratherapeutic levels
High-yield mini-pathophys
Calcineurin inhibitors increase vasoconstrictive mediators and decrease vasodilators in the kidney → decreased renal blood flow and GFR. It’s not “immune rejection” per se—it’s a drug hemodynamic effect.
Why Every Other Answer Choice Is Wrong (and what they’re trying to bait)
USMLE distractors are rarely random—they’re “neighbor concepts.” Here’s how to dismantle each one.
B. Sirolimus — Wrong (but know the signature toxicity)
Sirolimus (rapamycin) inhibits mTOR, blocking T-cell proliferation downstream of IL-2 signaling.
- Mechanism: FKBP binding → inhibits mTOR → ↓ response to IL-2 → ↓ T-cell proliferation
- High-yield adverse effects:
- Pancytopenia
- Hyperlipidemia
- Impaired wound healing
- Interstitial pneumonitis
- Key testable contrast:
- Not nephrotoxic (this is one reason it can be used as an alternative when calcineurin nephrotoxicity is a problem)
Why it’s wrong here: the vignette screams calcineurin toxicity (HTN, hyperK, high trough). Sirolimus doesn’t fit that renal vasoconstriction picture.
C. Mycophenolate mofetil — Wrong (think GI + cytopenias)
Mycophenolate inhibits IMP dehydrogenase, blocking de novo purine synthesis (especially in lymphocytes).
- Mechanism: ↓ guanine nucleotide synthesis → ↓ B and T cell proliferation
- High-yield adverse effects:
- GI upset/diarrhea
- Leukopenia
- Increased infection risk
- Pregnancy: teratogenic (commonly tested)
Why it’s wrong here: mycophenolate doesn’t cause the triad of HTN + hyperK + acute creatinine rise from renal vasoconstriction.
D. Azathioprine — Wrong (watch for xanthine oxidase interaction)
Azathioprine is a prodrug of 6-mercaptopurine, which inhibits purine synthesis.
- Mechanism: antimetabolite → ↓ DNA/RNA synthesis → ↓ rapidly dividing immune cells
- High-yield adverse effects:
- Myelosuppression (pancytopenia)
- Hepatotoxicity
- Classic interaction:
- Allopurinol and febuxostat inhibit xanthine oxidase → ↑ azathioprine/6-MP toxicity
- If combined, you must reduce azathioprine dose
Why it’s wrong here: azathioprine toxicity is hematologic/hepatic—not an early post-transplant hemodynamic AKI picture.
E. Basiliximab — Wrong (induction therapy clue + minimal toxicity)
Basiliximab is an anti–IL-2 receptor (CD25) monoclonal antibody.
- Use: induction therapy (early post-transplant) to prevent acute rejection
- Mechanism: blocks IL-2 signaling → ↓ T-cell proliferation
- Adverse effects: generally well-tolerated; can cause hypersensitivity reactions but not classic nephrotoxicity
Why it’s wrong here: basiliximab doesn’t cause the characteristic renal vasoconstriction syndrome. Also, the vignette points to a drug level–related toxicity, which fits calcineurin inhibitors, not basiliximab.
F. Infliximab — Wrong (autoimmune drug, infection/TB reactivation)
Infliximab is an anti–TNF-α monoclonal antibody used mostly for autoimmune conditions.
- Use: Crohn disease, ulcerative colitis, rheumatoid arthritis, ankylosing spondylitis
- High-yield adverse effects:
- Reactivation of TB (and other granulomatous infections)
- Serious infections
- Possible malignancy risk (lymphoma)
- Step tip: screen for TB and hepatitis B before anti-TNF therapy
Why it’s wrong here: infliximab isn’t a standard post-kidney-transplant maintenance immunosuppressant and doesn’t match the renal + electrolyte toxicity pattern.
High-Yield Table: Immunosuppressants You Must Differentiate
| Drug/Class | Mechanism | Common Uses | High-yield Adverse Effects / Clues |
|---|---|---|---|
| Cyclosporine, Tacrolimus (calcineurin inhibitors) | ↓ calcineurin → ↓ IL-2 transcription → ↓ T cell activation | Transplant maintenance | Nephrotoxicity, HTN, hyperK, neurotoxicity; tacrolimus: diabetes; cyclosporine: gingival hyperplasia, hirsutism |
| Sirolimus/Everolimus (mTOR inhibitors) | ↓ mTOR → ↓ T cell proliferation (↓ response to IL-2) | Transplant (esp if avoiding nephrotoxicity), drug-eluting stents | Pancytopenia, hyperlipidemia, impaired wound healing, pneumonitis |
| Mycophenolate | ↓ IMP dehydrogenase → ↓ de novo purine synthesis | Transplant, lupus nephritis | Diarrhea, leukopenia, teratogenic |
| Azathioprine / 6-MP | Purine analog → ↓ DNA/RNA synthesis | Transplant, IBD, autoimmune | Myelosuppression, hepatotoxicity; ↑ toxicity with allopurinol/febuxostat |
| Basiliximab | Anti-CD25 (IL-2R) | Induction in transplant | Generally mild; hypersensitivity |
| Infliximab/Adalimumab/Etanercept | Anti–TNF-α | RA, IBD, psoriasis | TB reactivation, serious infections, demyelination, HF worsening |
Exam-Day Pattern Recognition (what the question writer wants)
If you see rising creatinine + HTN + hyperkalemia in a transplant patient…
Think calcineurin inhibitor toxicity (tacrolimus/cyclosporine), especially if:
- Trough level is high
- Ultrasound shows no obstruction
- It’s early post-transplant
If you see delayed wound healing + hyperlipidemia…
Think sirolimus.
If you see diarrhea + leukopenia…
Think mycophenolate.
If you see pancytopenia + on allopurinol…
Think azathioprine toxicity.
If you see TB reactivation risk…
Think anti–TNF-α agents (infliximab, adalimumab, etanercept).
One Last Step-Style Takeaway
When a stem gives you a “transplant immunosuppression cocktail,” don’t just identify the class—map the adverse effect to the drug. Calcineurin inhibitors are the board-favorite because the complication is common, dangerous, and testable from labs alone.