A 55-year-old female developed ESRD due to poorly controlled diabetes. She underwent dialysis 2-3 times a week. She was advised for kidney transplant which she received from cadaveric donor. She was put on immunosuppressive therapy with azathioprine, cyclosporine, methylprednisolone and anti-thymocyte globulin. Two weeks following the surgery, the patient was discharged on medications. 2 weeks later she presented to emergency with complaints of decreased urine output. On examination, her BP was noted to be 160/95 mm hg. The region of the graft is enlarged and is tender to touch. Lab investigations revealed serum creatinine of 4 mg/dl. Her differential diagnosis includes transplant rejection and drug toxicity. Which drug could be held responsible for the above clinical scenario?

**Core Concept**
The underlying principle being tested is the understanding of **immunosuppressive therapy** and its potential complications, specifically in the context of **kidney transplantation**. The patient's symptoms suggest either **transplant rejection** or **drug toxicity**, both of which are critical considerations in post-transplant care.

**Why the Correct Answer is Right**
Given the clinical presentation of decreased urine output, hypertension, and an enlarged and tender graft, along with elevated serum creatinine, the most likely cause is **cyclosporine toxicity**. Cyclosporine is a **calcineurin inhibitor** known to cause **nephrotoxicity**, which can lead to decreased renal function, as evidenced by the elevated serum creatinine. This nephrotoxic effect can manifest as **acute kidney injury**, presenting with symptoms similar to those described.

**Why Each Wrong Option is Incorrect**
**Option A:** Azathioprine is an **immunosuppressive agent** that works by inhibiting purine synthesis, but it is less commonly associated with the acute nephrotoxicity seen in this patient.
**Option B:** Methylprednisolone is a **corticosteroid** used for its anti-inflammatory and immunosuppressive properties, not typically associated with nephrotoxicity.
**Option D:** Anti-thymocyte globulin is used for **induction immunosuppression** and is not commonly linked to the nephrotoxic profile presented.

**Clinical Pearl / High-Yield Fact**
It's crucial to monitor renal function closely in patients on cyclosporine due to its potential for nephrotoxicity. Adjusting the dosage based on serum levels and renal function can help mitigate this risk.

**Correct Answer:** D. Cyclosporine.

✓ Correct Answer: B. Cyclosporine