**Core Concept**
Pityriasis versicolor is a superficial fungal infection caused by Malassezia species, which thrive on the lipid-rich environment of the skin. Effective treatments for pityriasis versicolor target the fungi, interrupting their growth and replication.

**Why the Correct Answer is Right**
The correct answer is a drug that is not effective in treating pityriasis versicolor. **Option A: Ketoconazole** is an imidazole antifungal that inhibits the synthesis of ergosterol, a critical component of fungal cell membranes. This action disrupts the cell membrane's integrity, ultimately leading to fungal cell death. **Option B: Clotrimazole** is also an imidazole antifungal that targets the fungal cell membrane, although it is more commonly used for topical applications. **Option C: Selenium sulfide** is an antifungal and antibacterial agent that is effective in treating pityriasis versicolor by reducing the pH of the skin surface and inhibiting the growth of Malassezia species.

**Why Each Wrong Option is Incorrect**
**Option D:** None provided.

**Clinical Pearl / High-Yield Fact**
When treating pityriasis versicolor, it's essential to use a treatment duration of at least two weeks to ensure complete clearance of the infection.

**Correct Answer:** D.