**Core Concept**
Tacrolimus is an immunosuppressive agent used to prevent organ rejection in transplant recipients. Its toxicity can be precipitated by various factors, including interactions with other medications, which affect its metabolism or renal clearance.

**Why the Correct Answer is Right**
Tacrolimus is primarily metabolized by the cytochrome P450 3A5 (CYP3A5) enzyme in the liver. Certain drugs can inhibit or induce this enzyme, thereby altering tacrolimus levels and increasing the risk of toxicity. **Option C**, which is a strong inhibitor of CYP3A5, can precipitate tacrolimus toxicity by increasing its plasma concentrations.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect as it is actually an inducer of CYP3A5, which would decrease tacrolimus levels and potentially lead to a deficiency in immunosuppression.

**Option B:** This option is incorrect as it is a substrate of CYP3A5, and its metabolism is not affected by tacrolimus toxicity.

**Option D:** This option is incorrect as it is a CYP3A5 substrate, and its metabolism is not directly related to tacrolimus toxicity.

**Clinical Pearl / High-Yield Fact**
When managing patients on tacrolimus, it is essential to monitor their renal function and adjust the dosage accordingly, as tacrolimus toxicity can manifest as nephrotoxicity.

**Correct Answer: C. This option is a strong inhibitor of CYP3A5, which can increase tacrolimus levels and precipitate toxicity.**