**Core Concept**
Ketoconazole is an imidazole antifungal medication that works by inhibiting the cytochrome P450 enzyme system, particularly the 14α-demethylase enzyme, which is essential for the synthesis of ergosterol in fungi. This action disrupts the fungal cell membrane, ultimately leading to fungal cell death.

**Why the Correct Answer is Right**
The most serious adverse effect of ketoconazole is hepatotoxicity, which can be life-threatening. This is due to its ability to inhibit the cytochrome P450 enzyme system, leading to the accumulation of toxic metabolites, such as the toxic metabolite of acetaminophen. Additionally, ketoconazole can cause adrenal insufficiency by inhibiting the synthesis of cortisol, which is a critical hormone produced by the adrenal glands.

**Why Each Wrong Option is Incorrect**
**Option A:** While ketoconazole can cause gastrointestinal disturbances, such as nausea and vomiting, these are not the most serious adverse effects associated with this medication.
**Option B:** Ketoconazole can cause hypokalemia, but this is not the most serious adverse effect associated with this medication.
**Option C:** While ketoconazole can cause skin reactions, such as rash and urticaria, these are not the most serious adverse effects associated with this medication.

**Clinical Pearl / High-Yield Fact**
Ketoconazole is known to interact with many medications, including warfarin, cyclosporine, and theophylline, by inhibiting the cytochrome P450 enzyme system, which can lead to increased levels of these medications and potentially life-threatening adverse effects.

**Correct Answer: C. Hepatotoxicity.