**Core Concept**
Levamisole is a synthetic anthelmintic and immunomodulatory agent primarily used to treat parasitic worm infections and enhance the immune system. Its mechanism of action involves stimulating the immune system and modulating the inflammatory response.

**Why the Correct Answer is Right**
Levamisole works by stimulating the immune system through various mechanisms, including the activation of macrophages and the release of cytokines. This results in an enhanced immune response against pathogens. However, high doses of levamisole can have immunosuppressive effects, which contradicts its primary use as an immunostimulant. This paradoxical effect is due to the high doses causing an imbalance in the immune response, leading to immunodepression.

**Why Each Wrong Option is Incorrect**
**Option A:** Levamisole is indeed used as a single dose in the treatment of ascariasis, a parasitic worm infection caused by the roundworm Ascaris lumbricoides.
**Option B:** Levamisole is used as an immunostimulant, particularly in the treatment of various parasitic infections and as an adjunct therapy in cancer treatment to enhance the immune response.
**Option C:** Levamisole acts as a depolarizing neuromuscular blocker, causing paralysis of the worms, which is a key mechanism of its anthelmintic action.

**Clinical Pearl / High-Yield Fact**
Levamisole's immunomodulatory effects make it a potential therapeutic agent in the treatment of various autoimmune and inflammatory disorders. However, its use must be carefully balanced to avoid paradoxical immunosuppression.

**Correct Answer:**
✓ Correct Answer: D. Immunodepressant in high doses