**Question:** All of the following drugs are effective in the treatment of pityriasis versicolor, except:

A. Ketoconazole
B. Terbinafine
C. Clotrimazole
D. Fluconazole

**Correct Answer:**

**Core Concept:**
Pityriasis versicolor, also known as dermatophytosis, is a superficial skin infection caused by dermatophytes, particularly Trichophyton rubrum and Trichophyton interdigitale. The mentioned drugs are antifungal agents used for treating various fungal infections, including pityriasis versicolor.

**Why the Correct Answer is Right:**
Option D (Fluconazole) is not an effective treatment for pityriasis versicolor because it is a broad-spectrum antifungal agent that targets yeast infections like candidiasis. On the other hand, A, B, and C are specific to dermatophytes, the causative organisms of pityriasis versicolor.

**Why Each Wrong Option is Incorrect:**

A. Ketoconazole (Ketoderm, Nizoral) is a broad-spectrum azole antifungal agent that effectively targets dermatophytes, making it a suitable treatment for pityriasis versicolor.

B. Terbinafine (Lamisil) is also an effective antifungal agent belonging to the azole class. It is highly active against dermatophytes, making it an appropriate choice for pityriasis versicolor treatment.

C. Clotrimazole (Mycelex, Clotral) is a broad-spectrum azole antifungal agent effective against a range of fungal infections, but not specifically tailored for dermatophytes, making it less effective in treating pityriasis versicolor caused by these fungi.

**Clinical Pearl:**
Fluconazole is a commonly used antifungal agent for candidiasis, while the correct drugs for pityriasis versicolor treatment are azole antifungals like ketoconazole, terbinafine, and clotrimazole. The correct choice depends on the specific fungus involved and the severity of the infection.

In the case of pityriasis versicolor caused by dermatophytes, ketoconazole and terbinafine are the preferred treatments due to their superior activity against these fungi, while clotrimazole is less effective. Fluconazole is not specific for dermatophytes and is primarily used for candidiasis.