A 31-year-old male farm worker comes to the physician complaining of an itchy rash on his chest. Physical examination reveals an annular and scaling plaque five centimeters in diameter with central clearing on the chest. KOH preparation of skin scrapings shows branching septate hyphae, and topical application of terbinafine was prescribed. Which of the following mechanisms of action explains the antifungal activity of this drug?

Question

A 31-year-old male farm worker comes to the physician complaining of an itchy rash on his chest.  Physical examination reveals an annular and scaling plaque five centimeters in diameter with central clearing on the chest.  KOH preparation of skin scrapings shows branching septate hyphae, and topical application of terbinafine was prescribed.  Which of the following mechanisms of action explains the antifungal activity of this drug?

Quizplus · Accepted Answer

The patient described in this vignette is presenting with symptoms of tinea corporis. Clinically tinea corporis presents as an annular scaling plaque with well-demarcated, raised erythematous borders and central clearing. Terbinafine is a commonly used drug for treatment of dermatophytosis. It belongs to the class of antifungals called allylamines and may be used topically or orally. Terbinafine inhibits synthesis of ergosterol of the fungal membrane by inhibiting the enzyme squalene epoxidase. Ergosterol is a cell membrane sterol that is unique to fungi and does not occur in human cell membranes. This drug tends to accumulate in skin and its appendages, and its side effects are mild by the topical route. (Choice A) Binding to ergosterol is a mechanism of action of polyene antifungals (amphotericin B and nystatin). (Choice C) Caspofungin belongs to the echinocandin class of antifungals. It blocks synthesis of β (1,3)-D-glucan, a main component of Candida and Aspergillus cell walls. (Choice D) Griseofulvin binds to polymerized microtubules and disrupts the fungal mitotic spindle, thus preventing fungal cell mitosis. It accumulates in the skin and its appendages and is only effective in dermatophytosis. (Choice E) Flucytosine is an antimetabolite antifungal that is transformed into 5-fluorouracil in the fungal cell. It inhibits fungal protein synthesis by replacing uracil with 5-fluorouracin in fungal mRNA. It is used in systemic fungal infections. Educational Objective: Terbinafine is used for treatment of dermatophytosis. It inhibits synthesis of fungal membrane ergosterol by suppressing the enzyme squalene epoxidase.

A 31-Year-Old Male Farm Worker Comes to the Physician Complaining