A 58-year-old female presents to the Emergency Department with a several-day history of progressive, debilitating fatigue and generalized malaise. She reports new-onset dark, tea-colored urine (cola-colored) and a significant reduction in her urine output. She also notes new headaches and visual disturbances, which she attributes to feeling unwell. Her home medications include:

• Tacrolimus (trough level 10 ng/mL)
• Mycophenolate Mofetil
• Prednisone

Vital Signs at Presentation:

• Blood Pressure: 160/95 mmHg (new-onset hypertension, significantly elevated from baseline)
• Heart Rate: 105 bpm (sinus tachycardia)
• Respiratory Rate: 18 breaths/min
• Oxygen Saturation: 98% on room air

Physical Examination Findings:

• General: Ill-appearing, pale, and fatigued.
• Cardiovascular: Tachycardic, no jugular venous distention.
• Pulmonary: Clear lung fields on auscultation, no wheezing or crackles.
• Abdominal: Soft, non-tender, no hepatosplenomegaly.
• Neurological: Alert and oriented; no focal neurological deficits noted.
• Skin: Pallor noted; no visible petechiae or ecchymoses.

 Key Laboratory Findings:

Wrong Answer: A. Acute antibody-mediated rejection (AMR)

Right Answer: B. Tacrolimus-induced thrombotic microangiopathy (TMA) Rationale:

Key Clues: Triad of AKI + MAHA (microangiopathic hemolytic anemia: schistocytes, ↓Hb, ↑LDH, ↓haptoglobin) + thrombocytopenia in a CNI-treated transplant patient. The absence of DSA/C4d rules out AMR (A).

TMA Nuance: CNIs cause endothelial injury leading to TMA. "Therapeutic" tacrolimus levels don't exclude toxicity.

AMR (A) is a prime distractor due to hemolysis/AKI but requires DSA/C4d evidence. DIC (D) lacks fever/procoagulant labs (e.g., normal PT/INR).

De novo GN (C) wouldn't explain MAHA/thrombocytopenia.