**Core Concept**
Malathion is an organophosphate insecticide used for the treatment and prevention of lice and scabies. Its mechanism of action involves the inhibition of acetylcholinesterase, an enzyme responsible for breaking down the neurotransmitter acetylcholine.

**Why the Correct Answer is Right**
Malathion acts by irreversibly binding to the active site of acetylcholinesterase, leading to an accumulation of acetylcholine in the synaptic cleft. This results in overstimulation of muscarinic and nicotinic receptors, causing various systemic and peripheral effects. To minimize the risk of adverse effects, malathion is typically applied once every 7-9 days for lice infestations.

**Why Each Wrong Option is Incorrect**
* **Option A:** Malathion is not used once every 14 days, as this would allow the lice population to rebound and prolong the treatment duration.
* **Option B:** Malathion is not applied daily, as this would lead to excessive acetylcholinesterase inhibition and increased risk of adverse effects.
* **Option C:** Malathion is not used once every 3 days, as this would be too frequent and may cause cumulative toxicity.

**Clinical Pearl / High-Yield Fact**
To minimize the risk of malathion resistance, it's essential to use a combination treatment of permethrin and malathion, and to apply the treatment once every 7-9 days for optimal efficacy.

**Correct Answer:** C.