**Core Concept**
Permethrin is a topical **ectoparasiticide** and **insecticide** used to treat various infestations. It works by disrupting the **nerve cell membrane** of parasites, ultimately leading to their death. This mechanism is key to understanding its applications.

**Why the Correct Answer is Right**
Permethrin is commonly used to treat **scabies**, a skin infestation caused by the mite **Sarcoptes scabiei**. It is also effective against **lice**, making it a treatment option for **pediculosis**. Its use in treating scabies and lice infestations highlights its efficacy against **ectoparasites**.

**Why Each Wrong Option is Incorrect**
**Option B:** Leprosy is caused by **Mycobacterium leprae** and is treated with **antibiotics**, not permethrin.
**Option D:** Leishmaniasis is treated with **antiparasitic drugs** like miltefosine or amphotericin B, not permethrin.

**Clinical Pearl / High-Yield Fact**
Permethrin is a first-line treatment for scabies due to its high efficacy and safety profile, especially in **pregnant women** and **children**. Remember, permethrin is not effective against **eggs** of parasites, so a second treatment may be necessary.

**Correct Answer:** A. Scabies