## **Core Concept**
Crusted scabies, also known as Norwegian scabies, is a severe form of scabies caused by *Sarcoptes scabiei* var. *hominis*. Ivermectin is an antiparasitic medication used to treat various parasitic infections, including crusted scabies. It acts on specific neurotransmitter systems in the parasites.

## **Why the Correct Answer is Right**
Ivermectin primarily acts by enhancing the activity of **gamma-aminobutyric acid (GABA)**, an inhibitory neurotransmitter in the nervous system of parasites. By increasing the release of GABA and its binding to GABA receptors, ivermectin causes an influx of chloride ions into the neurons, leading to hyperpolarization and paralysis of the parasite's muscles. This results in the death of the parasite.

## **Why Each Wrong Option is Incorrect**
- **Option A:** Acetylcholine is a major neurotransmitter in both the central and the peripheral nervous system of humans and many other organisms, but ivermectin's primary mechanism of action is not related to acetylcholine.
- **Option B:** Dopamine is another neurotransmitter involved in various functions in the brain, including mood, reward, and movement. Ivermectin's action is not primarily through dopamine pathways.
- **Option C:** Glutamate is the most abundant excitatory neurotransmitter in the vertebrate nervous system. While important, ivermectin's mechanism of action against *Sarcoptes scabiei* does not primarily involve glutamate.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that ivermectin is effective against a wide range of parasites and its use in crusted scabies is particularly valuable due to its oral administration and relatively good safety profile. However, it should be noted that ivermectin is not always the first line for all types of scabies and its use can be limited by factors such as pregnancy and certain drug interactions.

## **Correct Answer:** D. GABA.