**Core Concept**
Itraconazole is a triazole antifungal agent effective against a broad spectrum of fungi, including Aspergillus species. Its pharmacokinetic properties, such as high lipophilicity and bioavailability, contribute to its efficacy in treating systemic aspergillosis.

**Why the Correct Answer is Right**
Itraconazole inhibits the fungal enzyme lanosterol 14α-demethylase, which is essential for the conversion of lanosterol to ergosterol, a critical component of fungal cell membranes. This inhibition disrupts fungal cell membrane integrity, ultimately leading to cell death. Itraconazole's broad spectrum of activity and ability to achieve high tissue concentrations make it an effective treatment for systemic aspergillosis.

**Why Each Wrong Option is Incorrect**
**Option A:** Ketoconazole, while effective against some fungal infections, is not the preferred treatment for systemic aspergillosis due to its potential hepatotoxicity and lower efficacy compared to itraconazole.
**Option C:** Fluconazole, primarily used for treating candidiasis, has limited activity against Aspergillus species and is not the preferred treatment for systemic aspergillosis.
**Option D:** Flucytosine, an antifungal medication, is often used in combination with other agents for treating systemic fungal infections, but it is not the best choice for treating systemic aspergillosis due to its narrow spectrum of activity and potential toxicity.

**Clinical Pearl / High-Yield Fact**
When treating systemic aspergillosis, it's essential to consider the patient's underlying immune status, as well as any potential interactions with other medications. Itraconazole's efficacy can be enhanced by administering it with food to improve bioavailability.

**✓ Correct Answer: B. Itraconazole**