An example of a systemic antifungal is:

Amphotericin B is classified as a systemic antifungal because it is used to treat severe fungal infections that affect multiple systems or the entire body. This medication is particularly effective against a wide range of fungi, including those that cause serious conditions like cryptococcal meningitis and invasive aspergillosis. It works by binding to ergosterol in the fungal cell membrane, creating pores that lead to cell death.

In contrast, the other options primarily represent topical or localized antifungal treatments. Clotrimazole and nystatin are often utilized for treating superficial fungal infections, such as those affecting the skin or mucous membranes, and under normal circumstances, do not reach systemic circulation. Undecylenic acid is used primarily as a topical application for conditions like athlete's foot and is also not systemic in nature. Thus, Amphotericin B stands out as the appropriate choice for systemic antifungal therapy.