**Core Concept**
The question tests knowledge of **Echinococcosis**, a parasitic infection caused by the *Echinococcus* tapeworm. The disease is characterized by the formation of **hydatid cysts** in various organs.

**Why the Correct Answer is Right**
The liver is the **first major organ** encountered by the parasite after it enters the bloodstream via the intestinal wall, making it the most common site for hydatid cyst formation. The liver's **reticuloendothelial system** and **Kupffer cells** try to filter out the parasites, leading to cyst formation.

**Why Each Wrong Option is Incorrect**
**Option A:** The lungs can be involved, but they are less common than the liver.
**Option C:** The brain is a rare site for hydatid cysts.
**Option D:** Other organs like the spleen and kidneys can be involved, but are less common than the liver.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that **liver ultrasound** is often used for diagnosis, and **albendazole** is a common treatment for hydatid disease.

**Correct Answer:** D. Liver.