**Core Concept**
The **FIGO staging system** is used to stage gestational trophoblastic disease, including choriocarcinoma. This system helps determine the extent of disease spread. Choriocarcinoma is a type of cancer that originates from the trophoblastic cells of the placenta.

**Why the Correct Answer is Right**
Choriocarcinoma with metastasis to the lungs is considered stage III according to the FIGO staging system. This is because the disease has spread to distant organs, but not to the brain or liver, which would classify it as stage IV. The **lung metastasis** indicates a more advanced stage of the disease.

**Why Each Wrong Option is Incorrect**
**Option A:** Stage I is incorrect because it refers to disease limited to the uterus.
**Option B:** Stage II is incorrect because it refers to disease extended to the vagina and/or pelvis.
**Option D:** Stage IV is incorrect because it refers to disease extended to the brain or liver, or both.

**Clinical Pearl / High-Yield Fact**
It's crucial to remember that the **FIGO staging system** is specific to gestational trophoblastic disease and is different from other cancer staging systems.

**Correct Answer:** Stage III.