**Core Concept**
Scabies is a parasitic infestation caused by *Sarcoptes scabiei*, a mite that burrows into the skin and triggers intense itching. Treatment involves killing the mites and their eggs. Among topical and oral agents, **ivermectin** is the only widely accepted **single oral dose** agent for scabies, especially in widespread or refractory cases.

**Why the Correct Answer is Right**
Ivermectin is a macrocyclic lactone that acts by inhibiting glutamate-gated chloride channels in the mite’s nervous system, leading to paralysis and death. It is effective against both adult mites and larvae. In scabies, **a single oral dose of 200 µg/kg** is sufficient for most patients, especially in endemic or resistant cases. It is particularly useful in cases involving immunocompromised individuals or when topical treatments fail. The drug has a favorable safety profile and is recommended by WHO and dermatology guidelines.

**Why Each Wrong Option is Incorrect**
Option A: Permethrin is a topical agent, not oral, and is not administered as a single oral dose.
Option B: Retinoids (like acitretin) are used for psoriasis or acne, not scabies; they do not kill mites.
Option D: Co-trimoxazole is an antibiotic used for bacterial infections, not parasitic infestations like scabies.

**Clinical Pearl / High-Yield Fact**
Ivermectin is the **only single oral dose** antiparasitic agent approved for scabies; all others are topical or ineffective. In mass outbreaks, it is preferred due to ease of administration and high efficacy.

✓ Correct Answer: C. Ivermectin