**Core Concept**
DEC (Di-ethyl-carbamazine) is a microfilaricidal agent used to treat filarial infections, particularly those caused by Wuchereria bancrofti, Brugia malayi, and Loa loa. It works by causing the death of microfilariae, thereby reducing the parasite load and alleviating symptoms.

**Why the Correct Answer is Right**
DEC acts by binding to the microfilarial cuticle, causing it to become more permeable and leading to the influx of water and subsequent death of the microfilaria. The mechanism of action is thought to involve the disruption of the microfilarial cell membrane, ultimately resulting in the death of the parasite. This effect is specific to microfilariae, making DEC a valuable treatment option for filarial infections.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because DEC is not typically used to treat trypanosomiasis (African sleeping sickness), which is caused by Trypanosoma brucei.
**Option B:** This option is incorrect because mebendazole is an anthelmintic used to treat infections caused by nematodes, but it is not effective against filarial infections.
**Option C:** This option is incorrect because DEC is not used to treat toxocariasis, which is caused by the larvae of the Toxocara species.

**Clinical Pearl / High-Yield Fact**
DEC should be used with caution in patients with Loa loa infections, as it can cause an adverse reaction known as "Calabar swellings," which are localized areas of inflammation and swelling.

**Correct Answer: C. Filariasis.**